Thank you, so it’s what I thought (though I couldn’t have explained why it is so), I can’t add non-contiguous sections to the same partition. Fair enough.

I’ve been frustrated occasionally by my Debian install, so I might take the opportunity to switch to something else. Maybe bazzite? Would be interesting at least. I’ll copy my /home off first, and if I use LVM again I’ll leave some empty space before it next time.