External Lacie BLu Ray Drive no longer readable by Mac Mini Sonoma.

I can't offer you much assistance but knowing how you made the backups might help. You mentioned the backups were made a few years ago. So the questions are, were they made with TimeMachine, or some other piece of backup software. If the latter perhaps that software needs to be updated.

You also mentioned they were done on your MacPro. Do you still have that Mac? If so have you tried using it to recover the files?

Unfortunately some devices may not be compatible with an M-series Mac. I have encountered a few myself.

Did you need to install any special drivers to use the BluRay discs or to use the BluRay reader/writer with your old Mac?

I know a common problem with CDs & DVDs were how you burned them. If you used sessions and the sessions were not closed, then it could cause problems accessing the data on another system.

I don't know how you stored the data on the BluRay discs, but if it was not with a Mac specific file system on the BluRay, then you may be able to try accessing the data using a Windows PC which may better support BluRay discs.

There is also a possibility the discs have degraded or become corrupted over time which has happened with burned CDs & DVDs....especially the lesser quality media.