Updated from Lightroom Classic v12 to v15, hangs while rebuilding catalog

I updated Lightroom Classic to v15.1, and also installed Lightroom to check it out.

Now, when I start Lightroom Classic, it claims that my catalog is corrupt, then tries to rebuild it, and hands during the process. I've tried with recent backups and ancient backups; the latest version of the app claims they're all corrupt, which I find extremely unlikely.

I tried downgrading to the earliest I could get onlike (v14) but no luck. The same crash happens. I'd like to try going back to v12 but I can't find anywhere to download that online. Can anyone recommend a solution that isn't "make a new catalog and lose all of your collections for the last two decades"?

[moved from bugs to discussions according to the community rules - Mod.]