upgrading PHP 7 to 8

We're currently running pro edition 25.5.14009 on Ubuntu 20.04 LTS. 

The latest version of PHP in the Ubuntu repos for 20.04 is 7 (7.4.3-ubuntu2.29 to be specific)

We want to upgrade to Ubuntu 22.04, which also upgrades to PHP 8, but our tests using do-release-upgrade uninstalls php 7 completely, so we're worried about breaking things in our Observium install. What is the recommended method to upgrade to PHP 8 in this case? 

1. switch to standalone PHP repository and upgrade PHP to 8 on Ubuntu 20 first, then upgrade ubuntu to 22
2. hold back php so it stays on 7, upgrade ubuntu to 22 via do-release-upgrade, then install PHP 8
3. Let do-release-upgrade do whatever it does and then install PHP 8 on the other side of the upgrade

Couldn't find a doc that covers PHP upgrades specifically, only which versions are supported, so if this is answered somewhere in the docs, happy to have a pointer.