Sober (flatpak) should work for Roblox :) it uses the Android version with some fixes, signing in was a little jank when I tried it but after that flawless!

Did you manually partition? It sounds like when you installed Nobara the /boot/efi partition got formatted, a similar thing happened to me recently (just wasnt paying attention), I used a Windows install USB to get a command prompt and restore the EFI entry.