If it is not an open source emulator, it will never be added, Recalbox is an Open source operating system that only uses open source emulators, or whose authorization is officially given by the developer.

the main problem could be the graphic power on the RPI 4, but we could try to run on software mode.

The problem really is the lack of power of the RPi4, I doubt that changing the emulator would solve this problem, so sorry to break your expectations, but I don't believe you will be able to play PS2 on Rpi4...