So I cleared out the low priority queue another time and left it for a couple of days (no changes since my last update) - it didn’t speed up immediately and had queued jobs piling up rapidly, but seems to have fixed itself given some time. The jobs processing is going blazing fast now. Using a 20s polling interval, seeing a range of 55 to 140 jobs per second over the last few minutes. Per day looks healthy too, no queue build up.
Regarding your questions, I’m not so sure how to check those. I’d be happy to check (would need some guidance) and provide the info if it’s still helpful though. Something possibly relevant is that I’ve had the ‘max emails per day per user’ setting set to 3 for a long time.