Very old plugin that I had from 2018 when I was learning Sourcemod. HL2DM does not really give you hints as to how to easily switch teams. This should solve this problem through commands or through a menu. Just updated it quickly. Match servers may have that, but for servers that do not ( looking at public servers ), this should be fine.
!teams/!switch - Will open a menu to switch teams. !rebels - Switch to rebels. !combine - Switch to combine. !spec/!spectate - Switch to spectators.
N.B. Requires cl_showpluginmessages to be to 1 for menus to show.
In theory, this should also work in any games that support the jointeam command.