I have a lot of questions but here are a few

1. There are bosses that are weak to sabers, but honestly instead of spending resources levelling Caesar (BTW Fergus is AOE) I'd just use a stronger one from the friend list like Okita, especially if you've already started levelling Gawain.

2. Camelot chapter 15.

3. Both are great but Euryale is better for Camelot specifically cos of her charm.

4. Both lancer versions are worth raising, but personally I'd only raise OG cu since he is more focused with what he does, and there's not much point to having 2 ST lancers who have similar functions.

5. Similar value (honestly neither are all that great), up to you. Personally I'd say Tamacat. The NP damage will be fairly close to Beo's, plus she can farm better than him cos AOE. The only downside is that stun.

1. Final 2 Fights in Camelot require a strong ST saber, but you can probably make crit Gawain work

2. Chapter 15? First Arrow, Your 2nd Fight against Lancelot

3. Euryale

4. If you really have no good servants
Cu(SN)>Cu(proto) for your lancer
Cu caster is inbetween
5. Np 1 Beowolf if you dont have a good st, Tama if you have trouble clearing waves

1: you can always rely on a friends saber for Camelot. So no
2???
3:For Camelot Eureal is the best.
4: Regular Cu is a good one to level
5???

1. For safety, if you don't have any ST saber Caesar and Bedivere is a solid Boss killer.
Caesar is the best budget option as you can summon him on the FP gacha, Bedivere is a bit expensive you need to raise his NP lvl by rolling from SQ gacha.
Gawain maybe AOE but he can hit very hard with his high atk and Buster deck. Can do farm, Boss killing, and Wave clearing.

2. If i'm not wrong or forget i get Mashu to 4 star in CH 15 node 1 Camelot ( everyone can correct me ). As for i remember she become gold not too far before facing Titoria. Likely when she face her father ( Lancelot ).

3. I would say raise for Robin, he destroy most saber, berserker and neutral enemy with his NP both female and male.
He is a Nuclear Warhead. I also have Euryale, she's strong for male killing but Robin is for general Boss killer.

4. Lots of lowstar worth raising, because they can clear any FGO content. Chu brother is a very strong example of stall team servant and backliner. But i prefer OG Chu, simple because he's tougher.

5. Depend on your needs, if you don't have any berserker ST raise him. Tamacat is a decent Farmer, she can clear High hp waves and farm with ease. I would recommend raise Tamacat first, she's NP 2 which mean she can deal more damage to multiple enemy and farm faster.
You farm everyday and use farm servant, it's her job.