Yeah, ideally you shouldn’t be tacking any characters on, but should have some diversity in characters as you’re devising them in the first place. I’ll just quote what I’ve said on this before…
I’d say it’s generally good to be including diversity as you’re creating the essential characters in the first place, rather than just tossing token characters in as afterthoughts, since the latter will tend to make them seem much less connected to the storyline… and it’s such a minimal effort that it’s not really much of a representation, either. It’s better if you’re thinking along the lines of “okay, I need a warrior who opposes the crown prince… ah, and the warrior’s spouse, maybe they can be lesbians” rather than “okay, I’ve written a whole bunch of important straight people, guess I’ll tack on a lesbian now!”

Also if you have set orientations then you have to deal with the Non-binary MC, which is going to be difficult. It will be very easy to seem like you assume the Non-binary MC’s gender/gender expression even if if you don’t mean to, no matter what you do here.
I’d generally go with determining this on a character-by-character basis, so you might have one character who goes for male MCs only, and one who goes for male and nb MCs but not female ones, etc. That way it just ends up being a matter of personal orientation… everyone’s different!

Honestly, I just want people to stop treating bi NPCs as though they’re “player-sexual”. Unless a writer has explicitly referred to an NPC as being that, there’s literally no reason to assume they’re not bi, pan, poly, or any of a dozen other orientations that express attraction to men and women ( and enbies of all sorts).
Yeah One thing that can make NPCs seem more playersexual is if they have an ex mentioned, or express attraction at some point, but it’ll flip so that their ex’s gender will match the MC’s, or they’ll express attraction to the MC’s gender only, and suchlike. I really wouldn’t advocate this… I find it erase-y.

Yep, as you’ve correctly surmised, I’ve gone for roughly 20% as what I (perhaps mistakenly) assumed was a “fair representation”, so I’m naturally concerned that I’m still missing the point here.
@ParrotWatcher’s covered the point quite well (thanks! ) so just to corroborate… there’s a difference between representation percentages in NPCs as a whole and percentages in romance options specifically. For characters who aren’t ROs, representation is still useful, but it doesn’t impact the player’s experience in quite as direct a way, and I certainly wouldn’t object to you going for the 20% there—that still means I get some representation, and could fit well
(and that’s the sort of thing that can work well in linear fiction, too.)
Romance options are different, because it specifically means that you would be creating a game that is less fun for gay players. Indeed, I can flat out say that I would not buy a game that features four ROs for hetero men and only one for a gay man. It’s just not fair to write a game that provides a worse experience for gay people. We already have real life for that.
You also mention you’re looking at the '20s LGBT scene, which does mean you have perfect reason to incorporate characters whose demographics aren’t identical to those of society as a whole anyway. (And, good on you for looking into that—there was a lot more going on than most people realize )
(Also, agreed on the issue numbers being difficult to report… it seems the more homophobia and heteronormativity are present, the smaller self-reported LGBT numbers will be… so it’s pretty well impossible to tell what the numbers would look like in a society without homophobia or heteronormativity, sadly )