All you say here makes me want to explode with deep comments. And, for the record, I have lots of shitty ideas. Many of the ideas I had for springkink & kinkfest prompts were shitty. Trust me. XD
Keeping to the story at hand, yes (!) I can see this as a much shorter piece that is a series of hard-punching drabble-like mini-scenes. Each scene makes a single point, all use evocative language, and at least half end with a gut punch. The work is in creating evocative prose and ordering the mini-scenes cleverly. We know this can work amazingly well in fanfic. (It doesn't in original fic, I would think?)
Going long (you already have 17,000 words) the challenge is whose story to tell. This story, via POV changes, elevates Balthier and Ashe to the level of having their own plot threads and Basch having a subplot thread. That means you are telling 2 and 1/2 stories (sort of). By story, I mean this: a character has a problem and he/she struggles with that problem, either succeeding or failing in the end.
Playing a math-game for the fun of it, 17,000 = 6800 + 6800 + 3400. Without doing something literary uber-fancy, that feels to me like just enough words to tell a very simple story for Balthier, a related simple story for Ashe, and weave in a single-themed Basch subplot. It also means that in order to show a character moving through a difficult transition, either the scenes need to stay short (ouch) or the transition needs to follow a very simple arc (also ouch).
Looking across other authors and my own writing experience, I find it hard to write a scene in less than 300 words. 1000 to 1400 is my personal comfort zone, which is around 4ish pages in a paperback novel. Chapters often contain a few scenes, but not always. Sticking with a basic three-act play structure, one needs a minimum of 3 scenes to tell a story (1. set up the problem, 2. show the conflict and confrontation, 3. provide the climax and the success-or-failure resolution). More scenes allows more subproblems, related problems, conflicts, etc. Therefore, 6800 words doesn't leave a lot of room for a lot of ideas. Call that (randomly) perhaps 5 to 8 scenes? That's just enough to tell a one-problem monomyth style journey. Of course, this is all bullshit-fudge-math because famous prize winning authors do whatever the hell they want and make it look good. But, rules of thumb. Rules made to be broken. But rules of thumb I use when planning (not always with success).
At 17,000 words a few rules of thumb (to be tossed in garbage quickly!) could be:
1. Just make this Balthier's story but hint at what Ashe is going through. Pop Basch in repeatedly so we know what's up later on. Reorder so the last scene is Basch speaking at Balthier's funeral.
2. Make it Ashe's story. Put heavier focus on her correspondence with Basch. Have a series of short scenes interspersed with Balthier's POV so we see how this is hurting him. Never used Basch's POV.
Or, make it much longer. Much much longer. With the ideas you list above, 30,000 seems like a base minimum and the scenes need to be tight. 40,000 gives you more breathing room.