I bet this is it. I had a 2018 Mac Mini with a failing drive that moved like frozen molasses, but wasn't throwing obvious errors. Before it failed, it was slow compared to an SSD, but booted up in a reasonable amount of time and ran office apps just fine, just with a little startup lag. It was bad compared to an SDD, but not intolerably slow.

If a Mac is running that slowly, there's probably a hardware issue.

Is there some reasonable way to check whether the Fusion drive is failing? Some quick searches suggest that Apple’s built in tooling doesn’t actually help much.

Sorry, I don't remember the details and it was several years ago. I do remember looking at the Console log and seeing lots of drive timeouts.