Run a rule on category change

As far as I can tell, it’s only possible for a rule to apply to an email when you either send it or received it.
I’d like emails to automatically be sent a specific folder as I give them a category.

You are correct. Rules only apply to new messages as they arrive in your Inbox, or on outgoing messages when you click on Send.

But you can always apply them manually at a later time. Just right-click on any folder and choose Apply Rules. Then select the Rules you want to use, and the folders you want to apply them to.

Why did you move this? It’s a feature request.
I am hereby officially requesting that category rules be a thing.

And applying them manually at a later time is no good, since it still doesn’t take just categories into account.

You can set your Rule to only apply to a tag, but you will need to apply it manually.

Right, I’m an idiot, I actually tested that before posting here.
It’s still not what I’m looking for however.