My issue with CM5 and SD cards failing has been resolved. I didn’t see much point leaving the long post about this here.
If you’re an idiot like me, see Rex’s post below for the solution and my reply explaining why it didn’t work when I tried it. My fault 
Reserved for any followup
there is an eeprom update for the CM5 that fixes a lot the sd card pickyness
I thought this might be the problem, but I checked and mine is on the latest version.
You need to edit it as per the instructions in the first post, it will solve your problems. You only need to do it once as the changes are saved to the CM5s firmware.
Hi Rex,
Ahh, I now know what I was doing wrong. I read somewhere that someone had solved the SD card issue by setting up the OS using the CM4 board and then switching to the CM5 once the OS was up and running with updates and what have you.
That was my mistake! I ran the commands for eeprom when the CM4 was installed, not realising they update the CM5 board directly at execution. So, although the config file was altered, the CM5 hadn’t had this instruction set written to it because it wasn’t present when I ran the job.
Thank you for pointing that out, I’ve spent a lot of time trying to fix something that was already sorted.
Still, I have learned a lot hehe.
All the best 
If the uCosole core is replaced with CM5, microSD card compatibility is an issue.
I have SanDisk and Kioxia cards on hand that are compatible.