Updated Catalina 10.15.6 Supplemental Update without any problems..
1. Using Clover ver. r5107
2. Did not update Lilu, WhatEverGreen or AppleALC
3. Updated to 10.15.6 (19G2021) from App Store.
4. Choose download and restart option.
5. The 1st restart took 1min , then screen flashed progress bar advanced 1/3 then restarted.
6. the 2nd restart took 3min, then screen flashed progress bar advanced 1/2 then restarted
7. The 3rd boot stated 43min to complete. ( Took about about 10 mins )
8. After 15 - 20 mins the 4th boot stated 10mins to complete. ( Be patient took every min. )
9. Then quickly booted to login screen with no issues.
10. Total process took about 20 minutes.
NOTE: Boots 1,2 and 3 need to boot from the correct disk ( Install/Data ) after each restart.