Hi Mario,
There is no customize button for Chromebook client as that for Windows client, so there is no way to uncheck "webrtc media optimization". I think you might disable the Teams optimization policy on the agent side instead.
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\VMware, Inc.\VMware WebRTCRedir]
"teamsEnabled"=dword:00000000
Then you won't see the "Media optimization" when clicking Teams avatar- About, Teams optimization will not work.