I have a decent understanding of flooring and painting now. 

If yours is the only STR within 20 miles and there aren't any cities or larger towns within a 20-30 minute drive, you could easily run into trouble even finding a reliable cleaning person, let alone a full-service STR management company.I would also definitely take the time to find some comps on Airbnb in the same area as yours, with as close to the same bed/bath count as yours, and with the same finishes and amenities as you are planning to put in yours.  

Hi, I just got into a fourplex property and found living room has a sagging floor.

I guess they did not clean the unit for over 3 years and did not call the old property management company when things started to break. 

Also if you have a clean unit the new owner will be excited about the idea of renewing your lease. 

Refinished hardwood floor underneath.

Can they be cleaned enough or is it replacement only?

A: "YES, PLEASE FIX THE SHOWER"A: "yeah ever since my landlord was tiling the shower and put some grout mixture down the drain we've been having consistent drain backups (2nd floor!)"

(Increased rates and loss of discounts on all policies, not just this one.)You’d be better off raising all your deductibles and using the savings to pay for your floor repair.

If you have carpet consider a cleaning clause if you don't have one.