They are almost certainly responsible for issues caused by abnormal use or abuse, such as a kid sinking his battleship into the toilet.

Handyman yes but for clogged toilets and such I will use a local plumbing co.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

I’m GUESSING it’s a toilet or underground leak, call it 90%. 

I do always recommend going with a property management company because I'm sure you definitely do not want to take 2AM calls from tenants about clogged toilets! 

Should I make an offer with the qualification that all furnishings, dishes, pots, and pans stay, except for "personal" effects?

We leave a starter set of essentials like laundry pods, toilet paper, paper towels, trash bags, dish pods and soap, etc and then we rotate out local goods. 

A few days ago, one of the units experienced a toilet backup.

I’ve fixed leaks, change toilets and still keep going up.I’m at appoint now when I moved out, I thought I’d be making money because while I lived there  I’d be going even.

If this endeavor doesn’t fully pan out, I’m comfortable treating it as a side project alongside my W-2 role.