I also keep hearing that STR is rather saturated overall, whereas midterm is chronically undersupplied.

With larger multi-units, there's a common issue I call the "chronic disease of a fourplex": - One tenant might leave within a couple of months, with or without a valid lease

I have some real life experience with this, my grandfather is 88 years old, his wife my grandmother is in failing health (chronic illness is expensive), he has 5 properties right now 4/5 were bought pre 1980 so obviously have appreciated a ton the 5th has probably gone up 5x since he bought in and that’s the one that’s done “worst” now this sounds like an great portfolio everyone dreams of right?

Housing will be a topic for decades and I don't see how we will get out of this chronic housing shortage any time soon.

In fact new construction in Milwaukee is almost non existant, so we have a chronic housing shortage (both for rent and for sale) but very little additional inventory.

That is true for many chronically understaffed county governments.

Chronically late, normally gets caught up, and takes pretty good care of the property.

I am a chronic saver/cash hoarder.

If its isolated extend grace but be firm so it doesn't become chronic.

Either way -- I suggest you read the individual leases (not just one as they may be substantially different) to see how you can be in a position to get rid of the chronically past due if needed as well as frame for a future need of getting rid of the gentleman who is "getting back on his feet" in the event that he stumbles again.Good luck!