Best Team for Thoma in Genshin Impact
Team #1: Thoma Burgeon
Team #2: Yanfei Overload
Yanfei's main source of DPS is her ability to trigger the Overload reaction, which does AoE Pyro DMG. This team and Overload in general is recommended only at early stages of the game, as transformative reactions perform better than amplifying ones (melt/vaporize) without proper late-game builds. Yanfei needs Electro characters to enable Overload and a shield, as she's pretty squishy, downside of being a Catalyst user.
Fischl comes with a considerable source of damage and energy generation, making her useful for both Yanfei and the other Electro teammate, which is generally energy hungry.
Beidou's Burst's coordinated attacks and Electro application provide huge DPS increase to the team. Beidou however needs another Electro teammate to keep her Burst active for as much time as possible.
Team #3: Xiao double Pyro
Xiao's main source of DPS is his Anemo-converted Plunging Attacks. The Anemo conversion given by Xiao's Burst makes him lose HP continously, so a Xiao team requires healing and shielding. In this team, Xiao takes advantage of Pyro resonance to increase his own damage.
Zhongli's role is to prevent Xiao's interruptions with his shield. Any other shielder does the job too, but Zhongli has the ability to keep his shield at 100% uptime and shred the enemies' resistances.