Is there a specific config that is executed when i join a game as a QTV spectator? If not, is it possible to make one somehow that automatically gets chosen when joining QTV?

IIRC no, but you could:

use alias my_qtv "qtv %1 %2 %3;exec qtv.cfg", then just type "my_qtv <params>"
OR
if you run qw client from browser (by auto opening of .qtv files), change assigned program from "client.exe" to something like "client.exe +<cfg_load|exec> qtv.cfg"