my plan to fix the issue.
Setup 1: GA P55A-UD4 + HD5770 already running on Yosemite:
1) download High sierra, use Unibeast 8.4, select legacy bios, install high sierra on a dedicated SSD
2) running High sierra, download Catalina, run Unibeast 10.1, select uefi bios
3) upgrade hardware to setup 2: Z390 + RX570
4) use the Catalina stick to install from scratch on the new system
why? this sentence:
Important: To get the correct installer, download from a Mac that is using
macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server.
https://support.apple.com/en-us/HT201372
I let you know if it's a success..