So, I’ve finally managed to upgrade all of my jails to 13.4-RELEASE after upgrading CORE to 13.3. I’m running supported versions of PHP and MariaDB for most things (still struggling with PHP on LimeSurvey, but working on it), so thanks again for the help and advice 
I still haven’t upgraded the pools as I was worried this might close off some options for me in the future, although I suspect they are too far away for me to be worrying about and I should just upgrade the pools as I would have done after everything was updated and working.
Is there any benefit to not upgrading to pools, or should I just do it?