Help！Need working 1.21.7 datapack or template for team-based minigame

I'm making a small 2-team PvP minigame in Minecraft Java 1.21.7.
Players should click an item (e.g. boots or bow) to join a team (Run or Shoot), then after both teams have at least one player, the game starts with a countdown and ends when one team is eliminated.
I've tried using custom items (with CustomModelData + display name) and functions, but many commands or NBT just don't seem to work in 1.21.7. I also tried using click detection but failed.
Is there any working datapack template for this kind of game?
Or is anyone open to helping me fix or rebuild a basic version?
Thanks a lot!