I'm trying to do something similar on a Asus TUF FA506.
This doesn't support legacy boot at all, but to allow it to boot a UEFI stick built by Rufus I had to turn of secure boot and fast boot.
This way the laptop does give me the option of booting Ubunut if I tap esc 5 times during boot or hold F2 during boot then F8 for boot menu.
I could also swap the boot order to make Ubuntu the default.
But, looks like 20.04 doesn't support the dual Nvidia\Radeon graphics as I just get a continuos Noveu (sp?) error message scrolling on boot.