Keep doing what you're doing and skip Rent to Own. If you need to relocate, find the cheapest possible rental and use the money you save to either pay off old bills or save for down payment.
Rent to owns are rare and more often a scam than a real benefit for the renter/buyer. Don't waste your time.
Find a lender who can assist you with credit repair. If you are in the low 600's you're close to getting approved. Some RapidRescore may be all you need to be able to buy.