Two installs, neither show up in the BIOS as bootable

Did you see this already?

I admit I only managed to dual-boot Windows and Ubuntu/Mint for about three weeks until the laptop decided not to “see” Windows any longer. I could never fix that and that was the last of Windows in my private life (still suffering professionally, though).’

Edit: despite it apparently being a security risk, I prefer Ventoy for my bootable USB stick, because you set it up only once and can then put several ISOs onto it and boot them all.