I'm a long-time fan on Windows, and I've just been trying to intro my friend on Mac Big Sur.
We installed the 7-day trial today.
She has two big folders of music - an iTunes/Apple Music folder of ~12,500 songs and a custom folder of ~2,500 songs.
With both folders added, the 15,000 came up ok, but mp3tag crashed on trying to use Search
On a second attempt, with just the 2,500 added, search works fine, but adding in the extra 12,500 from iTunes/Apple Music folder of Music, then again mp3tag immediately crashed on trying to use Search.
This time we did send the crash report off to the Apple Store when asked, so I assume you should have received the crash report from Apple Store.
We were so excited to be able to use mp3tag on Mac Big Sur and really hoping that the crash report will help you identify what the problem is and be able to fix it on the next release.
If you need more info, I can get back onto her Mac and do whatever you suggest and report back.
Many thanks indeed