Hello, costume, period and historical are some of my favourite genres. I'm fairly new to C-dramas, and I'm looking for the ones with intelligent characters, script and story. I'd also prefer a happy or decent ending, but I can handle bittersweet. 

Examples of what I've seen and liked:

Story of Ming Lan - This is just perfection.

Story of Yanxi Palace - I adore the FL, she's so refreshing. I love her attitude from the get-go.

Secrets of the Three Kingdoms - This one seriously went downhill, but it has many elements that I am looking for. There's no victim-of-circumstance damsel, the ML is refreshingly pure and kind-hearted, but also strong, and the power dynamics are interesting. Pity the lack of or unbelievable character growth for a number of the characters. The ending too, it's bittersweet, but has purpose and stays true to its characters.

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- Good characters is a must for me. They need to be intelligent, have dignity and backbone. Character growth too, I'm looking for stories that really let their characters grow. With female protagonists, I am very picky, I usually find them hard to relate to despite being female myself, but if they're remotely similar to the protagonist of Yanxi Palace, I will love them. I don't want any victim-of-circumstance damsel for a main character, please.

- I don't like romance, it needs to be a minor factor, and not a major plot driver. I hate romance in political settings, they're always unnecessarily angsty and irritating. If it's well written, I will like it, but I prefer to focus on politics and characters or family dynamics.

- Nice Sound. This seems to be a particular problem in older dramas, and is one of the main reasons I haven't seriously gotten into any yet. I'm a musician and always pay attention to the sound. If it's awful, I just don't get into it, but if it's beautiful, I seem capable of overlooking inconsistencies in other areas.

Decent ending. I like happy and can handle bittersweet, if it has a purpose. But if the writers choose to kill off the main characters as a final bid to rouse emotion I will be deeply unamused. I detest senseless tragedy, though tragedy with sense is another matter.

- No cultivation/immortal sect fantasies. I don't like them. I just, don't like them, so please don't recommend anything like The Untamed, Ever Night or The Legends. I'm putting this here, because some people still seem to recommend such dramas regardless. I'm specifically not looking for them. I want historical dramas, first and foremost.

- Minimal  to no comedy. I have a very serious sense of humour, and comedy in C-dramas, without fail, falls flat and exhausts me. I can't handle silly, "light-hearted" characters. I'd go so far as to say I genuinely hate comedy. I prefer quite and solemn tones.

- Believable and convincing politics. Yes, epic mind games are fun.

- I'm fine with stories many people might call "boring".  In fact, I dislike unnecessary melodrama. There doesn't always need to be an urgent and pressing event, a story which sets its own pace and refuses to be hurried is perfectly fine. All the same, I don't like length for the sake of length, and it is possible to draw things out far too long.

As a side note, yes, I am slowly watching Nirvana in Fire. I'm already aware of it. I hope someone can recommend something, I hope this isn't too demanding.

Since You already are starting Nirvana in Fire, try it has a season 2 also. Almost must watch..

I cant think of other ancient dramas aise from the ones you've mentioned, though maybe:

  • Legend of concubine Zhen huan
  • Legend of Mi Yue

but there are period (chinese republican era) dramas that are a must watch:

  • All quiet in peking
  • Battale of changsa
  • Auntie douhie
  • A touch of Green
  • The Disguiser

The Rise of Phoenixes  (Good politics and characters, but romance play a pretty important part here)

The Longest Day in Chang'an  (Good politics, but the drama is just too long for me personally, not sure if it suits you)

Young Blood  (Good characters, scripts, fast paced plots. Not that much on politics and is a half-comedy-half-serious drama)

The Vigilantes in Masks  (Good characters, scripts. Not very much into politics)

Joy of Life  (This is an upcoming drama, however, based on its trailer, it might be what you are looking for)

what you are looking for are this three  romance is not main plot:

  • nirvana in fire
  • princess agent
  • legend of mi yue

all this have  romance main plot

  • empress ki
  • general and i 
  • good bye my princess
  • legend of yun xi
  • princess silver
  • sound of desert
  • the princess of wei young
  • yanxi palace