I understand that you have already followed the WSUS maintenance guide and are still facing issues with the size of the WSUS database. One option you could try is to use the Server Cleanup Wizard which is integrated into the user interface to help you manage your disk space. The Server Cleanup Wizard can perform several operations such as removing unused updates and update revisions, deleting all client computers that haven’t contacted the server in 30 days or longer, deleting all update files that aren’t needed by updates or by downstream servers, and declining all updates that are expired by Microsoft.
It’s important to note that if you are using WSUS along with downstream servers, WSUS servers should be removed from the bottom up. Incorrectly running cleanup on any upstream server prior to running cleanup on every downstream server can cause a mismatch between the data that’s present in upstream databases and downstream databases. The data mismatch can lead to synchronization failures between the upstream and downstream servers.
If you have already tried using the Server Cleanup Wizard and are still facing issues, you could consider automating backup of WSUS database along with reindexing of the WSUS database on a schedule.