Actually, slight correction, at QF they train brace commands to be given until all pax comply, then stop. No point shouting thru an impact, you'll bite your own tongue off & be no use to anybody!
Brace commands are also only used at specific times and most likely not in an unprepared emergency where the first sign is hitting something.
Commands for doors differ on location & type of exit/door being used (e.g. overwing exits, slides/no slides etc) So while some standardisation could be done, it would still differ by aircraft type a carrier is using. Some of these commands have come about from prior experiences with different types of exits & doors actually used in an evac.