@CaseySJ , I know you have been very busy for the past 12 hours (as usual answering to all comments and requests
)
Did you have time to investigate the issue on NVRAM csr-active-config I mentionned earlier?
The value on your EFI folder #1 is E7030000, which enable to install V10 BIg Sur but prevent to reboot when install is completely done. Only Catalina booting with this setup
I found on another forum that the value should be 00000000, and this is confirmed on the Dortania guide.
No longer an issue for me as i installed both OSX and can boot on them. But it could be helpful for some other users...
Thanks again for all your support !