Morwen

There are two more story arcs before UN. I'm not sure if they were also filmed and just haven't been released yet (with these same actors here I mean).

Not exactly a sequel, because the last volume of the main novel series has never been adapted. There are a few flashback scenes that show important moments from that time, though, those are the ones that introduced Xu Zhenxuan as young Wu Xie back then (and that immediately convinced me that he's indeed perfect for the role).
Wu Xie and ZQL meet again later, before ZQL leaves for good and enters the Bronze Door once more, this time for ten years.
Wu Xie has to survive the insane world of grave robbers without him in that time and it's taking a heavy toll on him (he's also missing him, like, A LOT).
That's why Wu Xie is portrayed by much older actors later. Because when ZQL finally returns he's already around 40.

It's not in the book.

Watched the scene in which they find Wang Zanghai—or what's left of him—and the fact that the man has been dead for centuries and is STILL messing with them was just glorious.
I especially liked the bit where he smiles when Xiaoge bows to his corpse, and suddenly the whole room lights up. That was pretty awesome and really well done on a technical level.

I loved that part about them finding Zanghai in The Lost Tomb 2 as well, it was one of the more memorable scenes.

Very good! Very faithful adaptation. Excellent Iron Triangle, too, 10/10, great acting and great chemistry between the three of them, especially in the second story arc.
Would (and will) watch again.

The more I think about it, the more likely it seems. Maybe we don't have to wait so long for the next season after all.

That's true. Wu Xie has become the keeper of his memories (and not the first one, either, but the one closest to him in all those years) and records the newer ones for him.
So it's kind of like programming to ZQL, he will forget everything but the mission. But again, I can't quite remember the details. I REALLY recommend watching ABtBD, though. :)

... and now I'm at least 80% sure that they not only filmed more episodes, but maybe even filmed an entire second season already.
Why? Because of all the behind the scenes videos and photos in the snow.
We don't see the Iron Triangle climbing any snowy mountains at the end of this drama, so where do all those videos belong to? Not this season, that's for sure.
Which means there must exist more already.
And that would also explain the number of episodes in this season. Because 18 episodes is half a season, while 36 (or double the episodes) is more common for a full season.

Okay, so since I bought express and couldn't wait, I directly skipped to the end of the last episode first...

All those answers are in "Adventure Behind the Bronze Door", but I admit I would have to watch it myself again first because I can't remember all the details anymore.

But this condition of forgetting everything and only living for the mission is something that only the patriarch of the Zhang family has to suffer through. That's why ZQL—being the last patriarch—has it, too.

I think that's the beauty of the Iron Triangle's relationship.
No one really made the effort to stay friends with ZQL for long (because of the constant memory losses) until Wu Xie came along and tried again and again and again, until he became such an integral part of Xiaoge's life that he didn't forget him even when he lost all other memories again later.

It is explained, yes. And he's not exactly immortal, he just ages very, very slowly. Also, he's not the only Zhang to do so, there are at least half a dozen of them (some of them will only appear for the first time in the upcoming dramas).
The only one who's more prominently featured is Zhang Rishan (played by Zhang Ming'en), who's had at least a cameo appearance in most DMBJ adaptations (including UN). He's been around since the beginning of The Mystic Nine.

This drama is a weird one, indeed.
It's not an official part of the DMBJ universe, but it kept enough story elements of it that it could very well be one.
The fandom also seems to be divided over this.

Oh, three? I only know of their cameo appearances in "Southern Archives" and "The Mystic Nine 2", but considering where "Time Raiders" ends, I have good reason to believe we'll get at least another season with them.

Yeah, his story is super depressing. You'll get to see glimpses into his childhood and youth, and those were not fun for him. But "Adventure Behind the Bronze Door" also has the best last episode ever of any of these dramas. Not going to spoil it here but it was SO good.
Zhang Kangle plays ZQL, while Wu Xie is played by Zhang Luyi. And the ever wonderful Chen Minghao plays Wang Pangzi in this yet again.

The beginning maybe, but the main story takes place around the same time as The Mystic Nine. Fo Ye/Zhang Qishan is already a well established name in Changsha at that time.

I made only one payment for the entire drama on the day it was released and I've been able to watch all the daily express episodes since.

I don't see that happening with the Ultimate Note cast, although I think (?) they were planning sequels back then and just never got around to actually filming them in the end. =/
I really hope they keep these boys here long enough to actually make it to the end of the main book series this time.

I get the frustration with the actor changes but I'd still like to recommend watching them in spite of that, because they're still really good. I also wouldn't bet on those sequels ever getting remade.
My personal favorite of the series is Tomb of the Sea, which sees both Qin Hao and Chen Minghao as my favorite older versions of their respective characters (Wu Xie & Pangzi). This one is dark though and Wu Xie is (has to be) pretty ruthless here.
I liked Adventure Behind the Bronze Door a lot as well, this one is for you if you want to know more about Xiaoge's background story. I cried my eyes out repeatedly watching that one.
Reunion is also a fun watch and a lot of people absolutely love it. It wasn't my cup of tea, personally, but the acting is top-notch.

Absolutely agreeing with you here.
That would also solve the constant confusion about what comes when.

It's a new - and more faithful, imo - adaptation of the book series and covers the events of "The Lost Tomb (2015)" as well as the beginning of "The Lost Tomb 2".

As far as I understand it, it was made with heavy involvement of the original author himself and the plan is to adapt the novel series once again but with a fixed cast of actors—and, if we're lucky, to close the currently existing gaps in the narrative (like what happened after Ultimate Note/the 10 Year Promise/etc).

No to the first, but yes to the second.