Who to upgrade next?

I say get all your SSRs up and running just because they’re so rare. You hit that 1% for SSR and even less for that servant so make use of them!

You seem to have a lot of leveled arts servants so definitely Archer Artoria, Nero, and Nero Bride as soon as you can to widen your arts servant options. Helena is secondary for her team wide buffs. Everyone else after that in any order you want I’d say.

Don't neglect to level up some lower tier servants. My husband's account has a lot more 5* servants and he focused on them primarily. But they are so expensive to play you can't field them and all the 5* event CE's, but if you're 3 and 4* servants are low level, you have to choose between expensive leveled servants that can win vs expensive event CE's for drops on cheaper servants that struggle with the quest