Preferred sequence to upgrade all Stretch servers to Buster, 3.1.15 to latest 3.2

Should I upgrade all the servers from stretch to buster first, or better to do the 3.1 to 3.2 upgrade first. Or does it not matter?
If I do the Stretch to Buster upgrade do I need to do a ISPConfig update on each server after upgrading the OS or wait until I upgrade the entire system.

Trying to get my ducks in a row. Have server sprawl and have more then 20 servers to deal with. Wish there was a script that would just do it all
Thanks

 

I would update all servers to latest ISPConfig version, upgrade the os (do not forget to follow the setup-guide to install missing / new packages), run the ispconfig update for the server with reconfigure services and (maybe) adjust some settings in the interface for this server (php-version)