With a septic tank the only thing you should be flushing is crap and toilet paper - no flushable wipes (they don't go in sewers either, btw), no paper towels, feminine hygiene products, diapers, food from a garbage disposal, etc.

The problem also when soliciting bids, is different scope, you need to tell each and every contractor what you want, write it down, print it, give it to them for pricing, be as specific as possible, have samples from your local supplier store if possible, if not, let them know to give you a price limit on estimates, say "put up a budget of $150 for the toilets, ill pick a design at home depot later".  

., and if the old appliances we inherited go out, or they're having plumbing issues and I see that the toilet is older than Adam, I'll offer to put a new one in.

New washer and dryer, toilet and garage door opener.

Right now it's a gradual shift and I find myself hiring out more and more maintenance/repair chores (if I never change another wax ring on a reeking toilet it'll be too soon!).

It would be mailbox $ so you would not have to worry about tenants toilets or termites.

But I'd default to whats in your lease before worrying about local guidelines personally, especially if they are already out of the unit, might be worth getting those locks changed again asap so they can't get back in and squat.

And those ROIs can be consumed by one person breaking in and stealing a furnace, or squatting and it taking you months to get them out.  

You on the other hand, likely sell mostly to home buyers who don't know diddly squat about real estate, they're just looking for a home to live in.

For example, spending 2 hours fixing the toilet in one of your self-managed rentals almost certainly qualifies.