Well that's a depressing read 
but thanks again.
I actually got much further with UTM. Installing Ubuntu 22.10 was very easy (but a somewhat different experience to Fusion). Connecting a gdb client VM was easy. I can read source line by line etc etc. But ... no breakpoints. Sigh!
I've been having issues with gdb/kgdb for close to 20 years. My old colleague at VERITAS worked on getting patches into the Linux kernel many years ago. Obviously there aren't enough of us using this technology for it to have high enough priority.
I really don't want to have to buy another laptop. I'll keep exploring ...