1*~3* worth leveling for utility

For me, i know you're not new to the game and to be exactly a seasoned FGO player so i'm gonna straight to the point.

Here's some servants that i used pretty often and they're still very good after i've used them for a pretty long time :

- Caesar, you've Waver, Merlin and Hans mate ! Raise him already, because different buff stacks multiplicatively and he has all buff that you need. His skill is above lvl 6 ? Bring him to destroy everything and done. Don't bring him if you want some challenges, he's a monster of buffer.

- Asterios and Matahari, Buff stacking is not a new thing for seasoned or veterans so with your Waver, Merlin or whatever/any servants you use he is a behemoth for Offensive/Defensive gameplay. Usable for many kinds of playstyle for example Stall, OTK, sustained battle and more awesome way to utilize him.

- Blackbeard, almost the same like Caesar but only have one turn super strong steroid buff. Outside using him as your party buffer, you can use him everywhere as a farmer.

- i see you already have Saberlot and his skill level is high, raise Mozart for OTK Art Crit team or use him for general team guaranteed one turn Crit.

Lu Bu I guess. He has the strongest or pretty much the strongest NP damage out of all 1-3 stars and seems to compete with even 5 star servants for highest NP damage and Merlin should work great with him cause he provides the buster and attack buffs, stars/heals with Garden of Avalon and invulnerability with his 2nd skill. Seems like if you use double Merlin and him the damage he could put out with his NP or a crit brave chain would be insane.

I don't see these in your post.
Shakspeare 10/1/1 - Buster party buff (40% - 1 turn)
Mozart 10/1/10 - Art party buff (44% - 1 turn) & 50 crits stars
Alexander 1/1/10 - Quick party buff (20% - 3 turns)

These are my personal supports for each team since i lack both Merlin & Waver

To add:
Paraclsus - Has rapid casting, a minor 3 turn team arts up, and the rare skill to apply guts to a n ally(3 turns 1 time).
David - Charisma is always good and he has a 1 hit team evasion. Saved my ass alot in when I was starting.

Spartacus is a decent farmer. I use him together with Arash in my 3-turn farming setup.

Kotarou is a much better option than Cursed Hassan if you're solely looking for stars rather than additional survivablity.

David and Caesar are great supports and pack some punch as well. I see you already have Tristan, but David can also raise the party`s attack for 3 turns which Tristan cannot. So he can still be useful.