Forgive me if I have farther misread you at all in my response, but the reponse is made nonetheless.

I would like to hear your argument for what dramas with second seasons you think have truly justified it narratively, and also for why you think that the “one and done” formula is responsible for kdramas losing the plot halfway or not sticking the landing?

Because,
1. I do not believe the One and Done formula is at all responsible for the issues Kdramas have that you have highlighted, or any more besides, and I feel it is getting blamed for something that isn’t its fault.

2. I do not believe that adding second seasons is going to magically change said Kdrama issues highlighted.
There is ZERO guarantee that a second season, a split season or anything else will result in a better story or better direction. It’s like the argument that preproduced dramas are more likely not to implode as well. (They aren’t.)
The things that cause bad directing, bad writing, weak endings, and plots to implode are not directly correlated to a show’s length. And if anything, I think that some issues are better, not solved per se, but better addressed by going the OTHER WAY, that is by getting shorter, instead of longer.

3. I have always said this regarding episode length, drama length, and second seasons- that what is more important than anything else- or rather, what SHOULD be more important- is having a narrative length and style that is BEST suited for the STORY you want to tell, and what that story NEEDS.
If you do not know the story you want to tell, then that is primary problem.
If you know the story but not how to get there or how to end it, then THAT is your primary problem and it won’t matter how long or short your story is either.
*Controversial Opinion, but also, just because a creator WANTS something for a story, doesn’t actually mean it is what is best for the story, or what the story truly needs. A Creator might want another season- that doesn’t mean they should get it… because not all writers/PDs understand what their stories NEED, or indeed even understand storytelling, and then THAT is the primary problem.

As much as I am a big supporter of the Single Story, 12 Episode Format, I think THIS^ is more important than anything.

4. However. On the same note, I do actually think, that what most stories I have seen NEED, and would be best SUITED for, is NOT to be longer, and not to be split up, and not to be continued.
Because I happen to think that the One and Done format has actually always been one of Kdramas greatest strengths, and that they should be focusing on getting better at THAT and what that would truly need and look like, rather than throwing out that formula altogether.
(The One and Done formula is really strong thematically, for example.)

5. I can only personally think of two kdramas that I know of, and one is not even one I have seen, where a second season could be argued to have been beneficial to...

... the story, or added to the story, or made the story truly better in some way.
One is Forest of Secrets, which I have yet to see.
And the other is Age of Youth. (However, I think the second Age of Youth had both all the same issues as the first one, and also made certain decisions during its run which could be argued against it even being included.)
I have not seen a single other drama that HAS a second season or is GETTING a second season, where a second season is either good, adds to it or makes it better (planned or unplanned), or is truly justified or “artistically beneficial” as you put it, not even Kingdom (Patented Minority Opinion yes, and also, sorry Kingdom). I have not seen or heard of a single seasonal “split” that was narratively or artistically justified either.
And I’m not even sure that I can right now think of any show that doesn’t have a second season and that I truly think deserves one or could do with one (and even if it did get one, there’s no guarantee it would be good anyway).
And whilst there are plenty of shows that are longer and do justify being longer, none of them were split into seasons to do so.

6. But more importantly,
The Episode Runtime, the Number of Episodes and now, even the number of “seasons” and “parts”, has almost never actually been dictated by what the story needs/what a creator wants*- it is dictated by sponsors, advertisement, stations, ratings, viewership, and what airing slots are available, both by the day, and by the slot available over the year’s schedule.
None of which are Story First orientated.

7. So I’m sorry, but I will continue to be wildly cynical about Second Seasons, and similar what have yous.
I am not ANTI Seasons in theory (indeed, one of my own main writing projects would actually be best suited for a limited but multi seasonal run, and is planned as one), but I have no reason to believe that Kdramas are going about them for any of the right reasons, or that in doing so it will help them get better, because all evidence I have seen both in Kdramas, and in fact in Western shows, is overall to the contrary.

Both the true issue that you are trying to highlight, and the solution, lie elsewhere.
As they stand, seasons create more problems than they do solve them.

(**Weak Hero MAY justify its planned second season, but considering the end and tone of the first one, I'm REALLY skeptical. I also don't want to watch S2 because of how S1 ended. So that's not a good place to be in... if you fix it, you might justify it, maybe, in part, if you don't, then you will not.

I am also not including things like the ongoing School or Reply series.)

What other Manchu invasion dramas are out there? I remember the movie War of the Arrows, but apart from Chuno flashbacks I have not seen drama treatment of this event. There is the movie The Fortress, but I read that there is too much Injo in it, so I am reluctant to watch it.

It will affect viewing experience but means sustain subscriptions in the interim for the streaming platform. And potentially early paycheck for a partially completed series.