I've bought 2 old ThinkCentre desktops and they both adored Solus (plus they were fantastic products). They were early 2000s models and both had a mobo factory-named Dory. No kernel update ever effected them and they performed like Solus was born to be there. It was UEFI I believe.
I have to assume evert you have a newer Lenovo? It does not seem odd to me that a boot menu might have hid itself or changed the way it got called up.........no........what blows my mind is you say that you have BIOS access and there is no USB option in the section where you can manually change the boot order....
that's too weird to me in this foul year 2022...
unless you are saying there is no BIOS section regarding changing manual boot order?