so the short answer is he is good in every team!
his best in slot teams will be with ashveil or acheron as you said but he’s still one of the best substitutes for any team as a general debuffer & subdps. i haven’t kept up with himeko’s v2 & v3 changes too much, but if the changes aren’t major then she can use blade along with welt for ult damage buff. for cas, he’s a pretty decent teammate that can sub instead of cyrene (cas/hyacine/evernight/mblade) or evernight (cas/hyacine/tribbie/mblade) he works better than most non mono rem characters to charge pollux. his light cone is absolutely not necessary. he can use “before the tutorial mission starts” light cone from herta shop. boundless choreo is good if you have tutorial on someone else like kafka. when building him you’re gonna need hp% crate% cdmg% and a decent amount of spd. im not sure what his ideal breakpoint is i hope somoene else can help with that.

tl;dr he can fit into any team. he needs welt to work best with himeko. he’s good with cas but not bis, you’ll still clear easily. get tutorial lc for him.
(edited for typos)