Oh hey, now that I've been namedropped in this thread and am also on my laptop instead of on my smartphone, time to chime in with my own two cents, by which I mean several paragraphs of kind of unorganized semi-coherent trains of thought.
As the currently most mentally ill Fate tagger [citation needed], there are some obvious benefits to costume chartags. That being said, the argument for them should not start and end with "danbooru is a comprehensive database and we need costume chartags for every single copyright no matter what", because that is stupid.
I think combining similar-looking costumes with superficial differences is perfectly acceptable, and something we already do for FGO. Jeanne d'Arc (White Cruising) also encapsulates her similar-looking but not 100% identical Gravure Vacances costume from Fate/Extella, and Francis Drake (Stormy Seas Outfit) likewise covers the unnamed slightly different outfit she's seen wearing in the Atlantis Lostbelt. This is not a problem whatsoever, because if for some reason you actually care which pattern of bikini Jeanne is wearing or which eyepatched version of Drake you want, that is easily clarified with a two-tag search (costume/character + copyright).
And for Drake specifically there's also the Atlantis Lostbelt tag you can use instead, which is also what is currently being used for a lot of other alternate appearances throughout FGO--no individual tag for Sion Eltnam Sokaris as she appears aged down in Paper Moon, you can find what you're looking for with decent accuracy via sion_eltnam_sokaris paper_moon_(fate/grand_order). Same with the recently revealed younger version of Vritra in the current event, currently being covered by vritra_(fate) indra_no_dai_shiren. And so on and so forth.
Are all of these searches 100% accurate? No, but neither is any other search on Danbooru, including ones for costume tags. Just try looking through xiang_yu_(fate) solo right now, see how many of those results are actually for Yu Mei-ren, especially her lancer version with the character hair ornament. If there's only 1 or 2 false positives per page for a given 2-tag query, then we've got a winner. (If you want images of Xiang Yu you have better luck just looking through his regular tag without the solo qualifier, there's less than 10 pages of posts with him so that's not an issue.)
One of the few things I course corrected on in this regard was the creation of the Lady Yu doll tag, so its easier to find actual posts featuring yu_mei-ren_(fate) xu_fu_(fate) paired up together. And even then people still tag Yu Mei-ren on new solo uploads of Xu Fu anyways!
All that being said, I must confess that I've been doing something of an annoying experiment spurred on by a lot of this discussion where I've been tagging individual ascensions for some characters who arguably don't need it. Fuuma Kotarou (Third Ascension) is actually useful, but fuuma_kotarou_(first_ascension)_(fate) and fuuma_kotarou_(second_ascension)_(fate) are basically the same. Are they two distinct costumes with individual names we could be applying to them? Yes, but does anybody actually care about searching for things that specifically?
Just because something can be tagged doesn't mean it should. Hence why we don't have tags for eyes, mouth, and arms, and why most font tags were nuked a while back.
If you're going to use FGO as your defense for very granular costume tags in that copyright or others, then I have to break the hard truth to you that I do not actually care for most of these granular tags and Fate as a franchise doesn't even have individual costume tags for every single slight variation. Otherwise, we would've split Nero Claudius (Modern Costume of Crimson) a long time ago, to distinguish between the very important difference of when she's wearing red boots and no hair flower versus sandals and a hair flower.