Think I may have found what is causing this. We had a few repositories that auto download/sync from github. The user's key who connects to github was missing so the downloads were giving errors. I think this triggers a memory leak for the php process. Now that I've fixed the key issue no errors now appear and phorge looks to be happier, i.e. memory usage looks minimal all the time (i.e. php-fpm processes using ~100MBytes).
I'm going to keep monitoring for a few weeks.
Here's an example from the log:

What is the normal memory usage for the update daemon during the repository update? It seems to be peaking around 8GB per process for our repositories. Maybe I need to add more RAM, i.e. 32 GB to satisfy this, but that's not easy on AWS.

OK, not the monitoring issue.

Appears to be internal monitoring software causing this (not phorge), thanks for the replies.

done

How do I do that?

We have a working solution. Attached. Problem is it needs the BaseURI hard coding (replaced ours with XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX in the attached code). Can anyone merge this into the baseline code?

Thanks for the confirmation!