Ahead

For the fan's evil plot just let it slide cos it is unnecessary too.

If you are thinking if you should watch or not, watch from ep 1-8 and then, you can end it as you like cos the rest are predictable and it is just old boring cliche of ML thinking best for FL from his perceptive only! Just remove that ML got into car accident from the story cos it is unnecessary. Then, you can watch this through from ep 1-8.
Humor, romance and bromance were pretty awesome although I gave 6/10 only for whole drama.

First of all, car accident was the most ridiculous ever for a person like ML! He is attentive and careful of his surrounding! How could you kill him like that in fantasy story? Make it a logical death or just let him live cos this is freaking fantasy!
Fantasy stories exist for happiness and wild imagination.
Real life and tragic stories are for sadness, troubles, unexpected difficulty and hard-work.
Writing a story in fantasy theme/setting and writing sad story that isn't even logical is just wrong!
===
ML = Sadist & FL = Masochist (Literally, that's who they are in this drama!)
ML leaving FL for her sake is so cliche, old, boring and mad cos he is going to die anyway, so why not tell her the truth? And, if she left you, accept it; if not, be with her till death. Why not let her decided instead of deciding all by yourself? That would have been better plot. (Now, ML thinking best for FL from his perceptive only! This is just one-sided!)
"You think, doing everything the best for your love is more important than what your love one wants?" There is no honesty and trust! That's just one way communication and that's what most closed minded people do! - I am sorry but that's the fact!
FL is so strong minded and open minded though, as she waited so patiently for ML's reason for doing terrible things to her. (This is never gonna happen in real life. Anyone would quit that kind of sick relationship. There are freaking over 7 billions people. Everyone is special if you can look forward or look to the future instead of sticking with a person who has terrible personality! Don't stuck yourself in the past or in terrible relationship!)
===
Humor, romance, bromance and jealousy are the only things I like from this drama. EP 1-8 after leaving out s***ty car accident and fan's evil plot is great.
The rest is pretty much one-sided overwhelming love from FL. ML only accepted cos she found out the truth and her love for him is overwhelming (Which doesn't exist in real life so why sad story? Want to be realistic but with unrealistic setting and love? Okay!).
I was already able to predict that FL will accidentally see ML vanishing and that's how they are going to reconcile. (This drama could have ended with just 10 or 12 episodes. You could skip all those and ended with mishearing 49 days with 49 years! This point was unexpected though.) Finishing 'script' in 49 days is a overkill!
===
Seriously, you could have save this fantasy drama by removing unnecessary sad plots and giving happy ending. This could have been 9/10 or even 10/10 but now 6/10 cos of illogical accidents and sad plots.
FL & supporting characters' personality are awesome except for ML & 2nd ML. (When will I see a drama where 2nd ML just stop liking FL and get together with his soul mate and become friend with FL & ML again later.)
I am sorry but I can't stop myself from ranting!

Main characters being supportive to each other is so good to watch! :D

Well, all the main characters are 15 or 16 or 17 years old so they are all going to be immature.
I have learned that I should stop watching drama where main characters below 20 years old. Main leads who made logical decision and good judgement are rare in drama. I hate the idea that I have to understand for main leads' personalities just cos they are young but it is the undeniable truth too. We all made stupid mistake and stupid decision.
I don't like to watch back on my childhood drama again now cos I might hate it. I will just keep those old drama as good memory. Lying to myself but not 100% true though. XD
Nowadays, I preferred to watch drama where main leads are between 25 to 35. That way I can rant if they did stupid decision or if the drama did illogical arc/story/events. I will learn and be impressed if they made genius decision (Twist and turn, idea outside the box that I didn't imagine.)

I hope Ashin became good later on and not dwell on revenge alone.

The scariest person is the one who is calm and tenacious to reach their goal.
That is Ashin in this drama.
The only downside to watching this drama is that it is too dark (Too little light to see some scenes clearly. I know, they make it to be realistic.) so it hurts my eyes!

Well, it can be defined as plot hole or one male lead went to two parallel worlds.
A went to B and C worlds. B is where FL has memory and C is where FL doesn't has memory and in the past.
This drama is not to be taken seriously and it outweighs in fun than few bad plot holes.
So, just overlook and enjoy, if you can. XD

TEL 1-3 is the drama I would recommend if anyone ask me to recommend Chinese Drama cos this drama gives so much enjoyment; at least, for me.
You are not gonna regret watching this but don't expect too much cos high expectation ruin most excellent dramas.
E.g., You were expecting to get gaming laptop but you got Mac. XD

I was quite confused when I started S3 so I re-watched S1-S2 quickly to know important elements again. Only then, I was able to enjoy the drama much better. So, review S1-S2 quickly and come back to watch S3. I know, I know, it is going to take some time.

You forgot that he was and still the crown prince so he didn't know how to chase after girl like a normal guy in modern time. He was always aggressive cos that's how he grew-up.
He knew that FL didn't like him in modern time so he can't trust her. But he should have told much earlier though. I agree with your 2nd point.
btw, put spoiler for your comment pls.

TEL 1-3 is the drama I would recommend if anyone ask me to recommend Chinese Drama cos this drama gives so much enjoyment; at least, for me.
You are not gonna regret watching this but don't expect too much cos high expectation ruin most excellent dramas.
E.g., You were expecting to get gaming laptop but you got Mac. XD

TEL 1-3 is the drama I would recommend if anyone ask me to recommend Chinese Drama cos this drama gives so much enjoyment; at least, for me.
You are not gonna regret watching this but don't expect too much cos high expectation ruin most excellent dramas.
E.g., You were expecting to get gaming laptop but you got Mac. XD

EP 1, 09:21 min - YouTube
'And there's only a few girls of the right age in the palace.'
I don't think FL was 14 years old. Mostly likely 16-18 years or 17 years old.
Anyway, the show should have mention characters age at least once or give hints.

Okay, not way younger. My bad.
These leads have similar age, probably around 18. Li Changge was 18 I guess.
The Story of Ming Lan (2018) - Both are wise (The best duo imo)
Princess Agents (2017) - Both but ML is wiser
Legend of the Condor Heroes (2008) - FL is wise
Legend of Fei (2020) - ML is wiser
The Eternal Love (2017) - ML is wiser
Rookie Historian Goo Hae Ryung (2019) - FL is wise
The King: Eternal Monarch (2020) - ML is wise

I have seen way younger character who are wiser in other dramas.
I understand why she was so obsessed with revenge cos she was still immature and had no experience. But many characters in the drama said that she was smart but she was reckless cos she got no escape plan.
ML having to go to jail for killing Crown Prince's playmate was the most ridicules one. They were solving problem in much complicated ways.

2nd leads love transition, I don't buy it. Mainly cos of how the story goes. All main leads acts very well. Acting: 9/10
Story: 6/10
Enjoyment: 7/10
Overall: 7/10
What I love the most: Love, faith and loyal
What I hate the most:
Making a mountain out of a molehill. After FL stopped obsessing with revenge (after contemplating at the temple), most problems weren't that hard to solve cos she had a clear mind.
Before she she just wanted to have revenge or save other without any escape plan. She was lucky to escape most of the time cos of her fate or good deeds or ML.

I only liked episodes 2 & 12-16. Even then, I have to skim through those episodes to finish it.

MyDramalist might need to add skim option so I can add drama that I skipped a lot and watched.
For me, Reply 1988 is the best and the only one that is worth it to watch without skipping.
1988 > 1997 > 1994
For 1994, I only skimmed ep 1-2 & ep 15-16. Then, I am so done.
After finishing Reply 1997, I have decided, I must never ever get interested in 'idol fanatic'. XD

https://www.youtube.com/c/AsiandramaPH/videos
Better learn Mandarin and watch Chinese Drama. I quit watching Chinese Drama cos their English Sub are bad.

Here come the disappointments:
EP 14 - Gallery Arc
EP 15 - Pigeons’ raid
EP 16 - ML’s mum got murdered
EP 19 & 20 - Saving FL and Han Seo Arc
---------------
Politicians not knowing the face of ML & FL. They should know the face of tenants too.