What archer(s?) should i focus on? if any at all?

It's time to stop

if you need a support archer level up david
but the lineup you have is more than enough

I'd say level David for sure. His skill set is really unique to him and fun to use because of the strategies you can run with him. You don't have anyone with his niche too.

If by Gil you mean the adult Gilgamesh and not the kid one, then you literally have the best archers in NA maxed already, so I see no point in investing in any more archers, to be honest. However:

Atalante* might not have good NP gain, but she's one of the best star generators in the game, so it'd be interesting to level her specially since you have GIl, since he benefits a lot from crit stars.

Emiya recently got his second Rank up quest and undoubtely became one of the best SR servants in the game. His NP is good at generating stars as well, and being a Buster one allows easier Buster chains with your Gil. Also, his triple arts deck could also benefit your Archuria to help them fill their NP gauge faster. In other words, he can be pretty versatile and fits well with your other archers, either being supportive or another damage dealer. Also, he's cool as fuck.

Euryale is good for challenge quests since she has multiple charms, additional damage against males, vampirism (reduces enemy's charges and increases her own NP gauge) and so on. She's a good investment if you want a ST archer, and she also benefits Archuria with her arts deck and NP.

I can't say much about Robin, Billy and David, but they aren't bad either. Billy is quick focused, while David has teamwide evade I think. Robin has an arts deck and NP.

It depends on what you want, really. I'd personally level at least Emiya since he has a lot to offer, but other than him I'd say you have more than enough archers already. Perhaps it's better to focus on a class you don't have that many servants.

II think you have more than enough Archer to carry you. Billy and EMIYA is crit beast that will hit harder than any of your leveled archers but you have Gilgamesh and Chloe to do that although at lower crit damage than EMIYA and Billy. For single target NP Artoria and Chloe will charge their NP faster than Billy, Robin and Euryle. You can level up Euryale and David for support and stall team I guess. Euryale can stall male servant while David got team wide evade, heal and atk++. So if its me I go David -> Euryale -> EMIYA/Robin -> Billy -> Atalanta.

Maybe you can fix some hole in your Chaldea, like supp/tank/another DPS ( ST/AOE role ) or some Backliner. If everything covered, then you're good.
Raising and give more care to your favorite servant is also a good choice.
I would say you shouldn't work for more archers, since that's some monster archers have been leveled/raised already.

Definitely raise david. His party evade is a lifesaver. If you can build a quick team and have star generators, raise billy. If you have a kscope or similar, raise atalante as well.