I think Okita is actually better because of the NP spam potential with her crits. Meanwhile, the 3 B deck of Musashi makes sometimes hard to NP multi-times, and also fot in more generic comps. Sure, the first skill helps A LOT the NP gen, but you are going to need to draw that 1 arts card…to me, she would be actually on par with Okita if she had the tipical saber deck QAABB.
That said Musashi is right now far more memeable vs harder content because of the lack of quick performance sustained supporting (Alexander and Helena are the only ones, soon, Wu Zetian in EoR 2 gacha), plus Memerlin. The quick-Merlin, Skadi is going to even the balance. Dual Skadi+Okita sounds more OP than Dual Merlin+Musashi (that quick performance up would give her insane stargen and NP gain, alongside the damage).
As for your servants:
Herc and Cu are for backline.
Enkidu, once he gets evade, and Cu alter are good for solo, or alternative backliners.
Saberliz is budget Musashi. Use if you don’t have space for many 5*s in a party.
Chloe can generate a lot of stars with her 3rd skill on 10 and her arts cards, to benefit EMIYA Hawkeye crits. Also, ST NP spam.
LancerLiz is semi-suport, pair her with JalterLily for that sweet 40% dmg up for 3 turns, plus the def-down. Or any female, really.
Casgil first skill makes all cards puke stars, he’s and ideal partner for Nursery, a crit Caster. Level both first skills. Also, they both cause AOE def-down. And that arts up…
Nito for farming hands and assassin only nodes. She’s not good in dealing dmg, even on NP5.
Carmilla>Emiyassassin, her NP gain is actually very good, her NP is very useful, specially with Merin/LancerLiz support. A arts servant with bad NP gain is a pain to administrate…
Vlad benefits from the arts up of Casgil, ofc
Kintoki is self-sufficient and works anywhere.
Li Shuwen has 1-turn skills with big CDs, and need many stars to work, try to put him with arts people with low stargen who can give him that. Or 2030 CE. Tamamo can help in reducing the CD, plus upgrading the dmg. Hans gives him stars and consistent buffs.
Altera can be used now as crit servant (star gathering+instant stars), just don’t pair her with Riders and Archers. Merlin is good.
ps: raise a Georgius (lv 65 10/10/10 is a immortal), a Hans (chap generalist support only problem is dies fast and causes so much few dmg) and maybe even a Caesar (he has poor hitcounts and generation status, yes, but is a good dmg support for anyone, specially Okita-same class and deck, some more stars on NP).
ps2: Fionn kinda sucks, once he gets Clairvoyance Beauty he becomes a okay nice support for arts crit teams, and in 3 years his chanced evade will have a sane CD and 20-30% NP charge.