hope someone can help since it seems I managed to mess up my computer for good

I added a new SSD to have a little more space, then when I restarted my computer I was faced by a black screen.
OK, probably moving around stuff messed up the grub installation, let's try to recover it. Tried with both boot repair and grub customizer, no joy.
Ok, I wanted to install LM 19.3 in any case, so let's get on with that. But install always failed because apparently there is an EFI partition that couldn't be mounted properly, and the process always hanged at the "Removing not nedeed software" stage.
So I tried to fiddle with the BIOS settings, and I guess this is where I did something wrong, like disabling and then re-enabling secure boot, because now the computer will just show 4 lines of text about stuff it doesn't find ("Something went seriously wrong": yeah, you bet!!!) and then turning it off all by itself.
Since this is my main work box, I really hope someone can explain how to fix it! Things were waaay easier with the old BIOSes

Many thanks in advance,
R