I think you can prioritize any program for quite some time on a Mac, no need for game mode to acchieve that.
Open a terminal.
Run ps -ef and identify which of the running processes is Photoshop. Take note of the process ID (PID).
Run renice -n -20 -p (put PID of 'Photoshop' here).
Be aware that maximum priority is set with -20 and minimum priority with +20.
nota bene: you could use nice to start Photoshop with higher priority with a shell script directly; as written renice allows to change the priority of already running processes. Boosting the bluetooth connection like in game mode seems unnecessary to me, obviously YMMV 😄