[Koha] Upgrade order

Hello the list We presently have a server with 5 instances, running 19.11.22 on Deb 9 (stretch). Over the new few months we aim to upgrade both - from stretch to buster, and from 19.11 to 20.11 or maybe 21.05. Which is the best order to do the upgrades in? OS first, or Koha? My natural caution leads me to prefer to do them separately. Tom Burke