Well, the good news is that I got it to work. I'm just...not quite sure how. 😅 Downloaded the full pkg from MrMacintosh, but before running that I thought I'd give Terminal one final go having spotted I used a '-r' as a reboot/restart command instead of '-R'. Which I think is wrong. 😳
All of a sudden, Terminal found 15.7.7 somewhere on the system (not the downloaded pkg, that was still waiting to be extracted in my Downloads folder), immediately rebooted and installed it. Re-ran root patches once the install was done, and here we are. 😀