Can't install libreoffice, unmet dependencies
Solution 1:
(summarizing comments)
The errors encountered when trying to install libreoffice were due to the focal system referencing a bionic libreoffice ppa.
Completely removing libreoffice, removing the ppa, and reinstalling libreoffice was successful, although the end result was an older version of libreoffice than previously installed.
There are other non-focal ppas on the system which are not currently breaking the package system but should probably be investigated in the future with an eye towards replacing them with the focal versions if possible.