That Retroflag GPI looks great, tempted to do something myself but this is the first I have seen them, have to have a read up as would like a little retro handheld but don't want to spend too much. SNES, Megadrive and gameboy sort of emulation
Get a Miyoo Mini Plus or the RG35xx. It will play up to PS1 perfect. There is a GPi2 Case with a slightly bigger screen, but I think it would cost more to build a GPi (factoring the need to the Pi Zero 2) than to just buy one of the other handhelds.