Just had a minor breakthrough that I wanted to share in case this works for someone else too (on a Debian Trixie from February, 6.18x).
I’d been yet again frustrated by my LCD screen, all morning. Loading overlays and what not. I’ve recently figured out how to compile them myself, (adapting the Makefile to Bash, to similarly run the C preprocessor to pull in the headers).
Anyway, to cut a long story short I realised our design had MOSI on D0 and MISO on D1, and I needed to enable this line (#57), just like the preceding comment suggested: