The usual Mail issues after update: slow, read message stays unread etc

After the update to 12.1 the usual issues emerge again when using Apple Mail.

Unread status does not change to read. The spinning rainbow wheel appears.

Now I have to go through the whole messy list of possible and tiresome fixes. Deleting envelope indexes in the library and such. I lost the post which instructs me precisely what to do and what to avoid. Could someone craft a good one again for me? And probably many others...

Why can't Apple make this into an app or a button? Or just prevent it alltogether while at it...

Many thanks in advance!