@leocg OK, partial solution update:

I've got my data back onto the cloud and the bookmarks & passwords have returned to my browser.

However, my 60+ tabs are still missing. I believe its due to the current browser session overriding it. I've looked through my history, which becomes incomplete prior to the reinstall.
Is there a way to copy the files from the old drive (or other retrieval method) pls?

@leocg Thanks, I'll try booting the old drive, unless I can transfer files?

@leocg So if I disconnect the new drive and boot from the old one, the data will still be gone?

After installing a new C: drive (I still have the old one connected as F: drive), I was trying to sync my data. I had no idea what my passphrase was, so I tried to reset it. I was too tired and couldn't see very well, and didn't properly read what I was doing, just clicking through the process - and Opera decided to NUKE my account.

Is there any way to retrieve my data pls?

Win 10 Pro
Opera GX version LVL 4 (core: 89.0.4447.64)