Servant suggestions

Alexander with his charisma and quick buff can help your Rider Kintoki and Ushi.

Only 3 at the moment

Medusa, a bit selfish with 2nd and 3rd skills only benefiting her. Her 1st skill stun needs the levels to be consistent but it's universal meaning all enemy types can get stunned. Shel can charge her NP reliably enough because of the 3rd skill but still needs a bit more NP gen on attack because of her low hit counts and NP gen overall. NP is damage is decent because easy to NP5 and put in a quick chain, overcharge works best in a quick team or at least those whose cards have a lot of hits on them.

Alexander, balanced quick support in my opinion. Charisma is on the same as Ushi, got a charm skill like Kid Gil with enough levels can work on Humanoids, as long as it resembles human you're good, and a party wide quick buff. Again being focused on a quick team but can work in buster quick post NP, overcharge 1 gives 15 stars. Certainly not much and given that he's a rider the stars will likely go to him so you need to have CEs that have star absorb or a servant who can absorb those stars via skills. You may want to use Hans 3 stars per turn plus crit damage increase skill if you don't have a gold Caster.

Blackbeard, Buster aoe rider. NP damage is ok, overcharge is meh you won't count on it even if it's the last NP in an NP chain 70% is the same as an NP1 overcharged Archuria. Skill wise he is good, 1st is like the 5 star Drake, gives NP damage and attack boost for 1 turn, you'll want to use this if you have multiple NPs ready. 2nd skill gives perma guts and 3 turn attack boost at the cost of being more prone to debuffs for 3 turns as well, the Guts won't expire until Blackbeard dies or you get buff removed. 3 skill is a decent heal but he's more of a female support as it gives twice the heal to females and genderless servants. Pair him with females, there are a lot of good ones and give him NP gen or flat NP CEs even with the balanced deck his NP gen is not good.

Literally just Alexander and Blackbeard. No one else has any team utility.

Boudica will be getting an NP interlude here in a bit that gives a real good team boost so that might be something to keep in mind too if you want an arts rider.

Like mentioned above, Alexander and Blackbeard is the most exact servant you're looking for. Yes, they pack a punch and their NP hit real hard.
Boudica is more leaning to Durable team set up.

Well IMO Ushiwakamaru is BEST 3 star rider who can actually support and pack a punch herself. And In JP , They decided to add sugar over honey by adding 3 turn Quick boost to Her Tengu Stgy skill. So she is worth the Investment.(Even Grails). And Alexender for AOE Rider with some mini supports work too. And Go for Medusa if you like her Animation(lol jk) or Doors farming.