What matters is I can put that exe on a harddrive/usb stick, plug it into any PC, and play it. Does that work with epic and steam? If I copy the game folder onto a usb stick and buy a new pc, can I plug it in and play it without ever installing the launchers? Or having to do some other workaround like download software I don't know if I can trust that I wouldn't have to do with GOG?

I don't care about whether it's technically illegal or not, I only care about how easily I can play the game using nothing but the game, exactly like a cartridge.