I have the same problem, but did manage to install 10.15.0 using that chinese guy EFI folder, my RX5600XT was not recognized because it started to have good support on 10.15.3. And unfortunately I didn't manage to upgrade that system.
Tried everything from the beginning with Olarila 10.15.4, EFI from 300 series, "agpdmod=pikera", and a custom DSDT provided by Maldon, this time I got the 2m mark problem, and booting from disk didn't help.
After that, I tried Olarila 10.15.3, with default and updated the kexts, all I can get is a reboot during install, and a shutdown if a try to boot from system disc.
If you guys find a solution, please share it back here, I don't know what else I can try....