Team Comp/ Building and Ascension Material farming

Well as far as gears and pages go you'll be able to get 20 of each in the upcoming journey to the west event, beyond that you can farm pages at Hyde park London, and gears at Chicago, America or Clerkenwell, London if you're not that far in the story(Chicago has a better drop rate).

For teams what you're looking for changes a lot from mission to mission but Robin Hood, Mozart, (support)Waver is a solid choice for high difficulty content only "straight" upgrade to which would be swapping Mozart out for Tamamo if you ever get her, and Euryale can sub in for Robin to kill males. Hans and a support waver can be put with basically anything with higher star weight than casters(Karna seems like a good choice from your servants especially since he has his own crit buff skill) and start doing insane crit damage. Heracles is a generally great stand alone servant too especially with his bond CE who you can throw into pretty much anything and he'll be at least all right. Other than that Cu is basically the best 3* in the game and should absolutely be leveled all the way he's the games f2p/c2p hero. Finally having a few taunt servants(Mash, Georgios, Leonidas, etc) leveled can be invaluable on some high difficult missions special mention to Mash as she costs 0 and has great supporting skills.

Ushiwakamaru + Hassan is pain, just waiting to happen. Grab a friend's Scathach or (strengthened) Alexander and watch the numbers fly.

Hans goes with pretty much anyone that he can help get Noble Phantasms off with. He'd work well in particular with Robin Hood, Mozart, or David.

Cu Chulainn is incredible at high levels. Support him with David, Mash, or Georgios, and he's nearly unkillable. Same can be said for Heracles.

Karna and Heracles are gonna be your big hitters. Caesar and Shakespeare can support them pretty well, while Mash/Georgios and David can keep them alive for tough fights.

Hope this helps!

Karna, Caesar and Hans could be a good frontline since Karna deals damage, Caesar helps with offensive buffs and Hans helps with supportive buffs.

Robin and Ushi are good alternative ST damage dealers and are worth the investment. Leonidas and Georgios are good tanks that can be useful in some situations, but generally less prioritized. Also don't underestimate Mash, as she is a very cost effective servant and can also be a very good defensive servant.

On the backline, Cu FSN and Heracles (lvl 10 bond only) is nearly unkillable so they are obviously good choices as stallers. Heracles also deals a lot of damage so alternately you can put him in the frontline if you don't have any better damage dealers. Hassan can also function as a staller but he only relies on his 3rd skill, so he won't be as effective.

Eternal gears can be farmed in America in maps where you fight robots (Chicago is the best node) and Pages can be farmed in London's Hyde Park. Note that drop rates may be atrocious, so be prepared to spend a lot of time and AP to farm them.