Well an ITX motherboard will fit an MATX caseYes, thanks. It's much appreciated. When you first posted, I tried to build around a 13700k, but struggled to find an equivalent mATX setup in my market. I'll look again today. I've decided to go with the smaller from factor for a few reasons too boring to list here, so that limits case and Mobo options.
The rm850x was on my shortlist before bed. I'll do some more research today and try replacing the PSU before buying the rest. No harm in trying.