A more varied servant list does make certain fights easier / faster, but given you made it this far without them, you can probably guess you don't need them.
If you want to get a meta support, my suggestions is Caster Artoria. Her next banner is early to mid next year, and she works amazing well with a number of your Arts servants. Combine her with your Bride Nero or Tamamo, you will have amazing support for most of your Arts servants , while still serving as a good battery support for Buster and Quick servants you might use.
https://fategrandorder.fandom.com/wiki/25M_Downloads_Campaign
AoE NP servants like the following would benefit by becoming capable of 3 turn loops to speed up farming or more simple battles.
Da Vinci (Caster)
Summer Musashi (Berserker)
Space Ishtar (Avenger)
Kiara (Alter Ego)
Summer Kiara (Moon Cancer)
Zenobia (Archer)
Percival (Lancer)
Habetrot (Rider)
Gilgamesh (Caster)
Mysterious Ranmaru X (Avenger)
Salieri (Avenger)
I want to mention Erice (Lancer) and Nitocris (Caster), who while great Arts servants, are heavily limited by their Instant Death NPs, which can easily ruin looping with them. Later servants with ID like Summer Kiara trigger after damage, so they get their full refund, while those two trigger first, meaning no refund.
The following ST servants would become insane boss killers with Artoria's support, clearing out full health bars with a single NP and repeating that for a few turns depending on how well you manage things.
Sitonai (Alter Ego)
Chiron (Archer)
Summer Anastasia (Archer)
Summer Corday (Caster)
Chiyome (Assassin)
Emiya (Assassin)
Astraea (Ruler)
BB (Moon Cancer)
Izo (Assassin)