I've reviewed the PR - could someone else have a look at it, too?

Later than planned, but here is a first try : https://github.com/ImpressCMS/impresscms/pull/1429 

That's certainly a viable solution. I'll take care of that this evening.

I was thinking along the lines of using the existing UA field and having it do some double duty. Do nothing and we assume, based on the format of the tag, that it's for UA. If you have switched to GA4, include the G- at the beginning of the tag and we'll load the proper snippet.

Using the preloads is a good idea - I think we should go that route for this round. The next phase will be to find a better way to insert and manage them from the control panel.

Short term : can't we simply add an extra field to the ACP where users can put their GA4 number? That would allow both Universal Analytics and GA4 to coexist for the time being.

The idea of custom tags it not bad, but configuration of these features should always be kept limited to the ACP, so even if you define a custom tag to include GA4 or Fathom or any other service, we will need to figure out a way to define these identifiers freely in the ACP as well.