Not sure if/how you can safely revert, other than restoring from a backup.
FWIW (as you probably already know): bookmarks in your "Favorites" main directory will appear as square favicons in the "Favorites" section on the Safari Start Page.
They're not as large or easily identifiable as the thumbnails in the former Top Sites, but it's functionally the same. AND you can add folders. You can also have more icons - 9 per row, and at least 5 rows. Maybe more? I haven't tried going past this yet.
I think they're mimicking the look/feel of the iOS icons or MacOS Launchpad. That seems like a valid enough design choice. It's just going to take some getting used to (at least for me).
Once I cull and re-organize my "Favorites" bookmarks, it will probably look and work better.
Just wish I could just make the icons bigger. (sigh)