Good idea or waste of money?

Would it be a waste of capital to replace the old and bring in the new?

It won’t have the said effect if you waste your time putting comps together for them. 

Besides the rent and the costs normally associated with move out (cleaning, paint, etc.) the tenants are also responsible for marketing costs (which are basically zero as I do all the work myself and do not pay to advertise) and commissions that have already been paid, but these again are usually very minimal fees that are not really making any of my tenants think twice about leaving early. 

I purchased with a VA loan so my down payment was very minimal. 

I would do everything I could to stabilize your free and clear properties for the near future so there are minimal cap ex. 

If you truly want to lock in a set monthly payment with very minimal increases in taxes and HOA dues, then I say go for it. 

They take on minimal risk and are getting guaranteed 10% returns with no work.

Your time would be better spent finding deals and networking than chasing down clogged toilets.Once you have a system in place with a good property manager, your time should be minimal to manage that property....1-2 hours a week.I hear you that you're burned out...totally understand.

If a house in on septic then buy an waste incineration system, they are available.