Can a buyer actually assume my existing mortgage rate when I sell fast

Your read on the mechanics is accurate. The due-on-sale clause means the lender can demand full repayment the moment you transfer title, so a simple assumption on a conventional loan is not available to your tenant no matter how strong her history is.

The two paths you named are the right ones to understand.

The first is a clean seller-financed sale. You sell to her, your existing mortgage gets paid off at closing from her down payment and whatever cash you bring or borrow, and then she pays you directly under a new promissory note at whatever rate you two agree on. You become the lender, she gets a rate below today's market, and you get a monthly income stream. The rate does not have to match your aunt's original rate; it just has to be something you both find fair.

The second is a wrap-around mortgage, sometimes called a wraparound or wrap. You keep the existing loan in place, you create a new loan to the buyer at a higher rate, and the difference between what she pays you and what you pay the underlying lender is your spread. The problem you already spotted is real: if the lender sees title change and invokes the due-on-sale clause, the whole balance comes due immediately. That is not hypothetical exposure, it is a specific legal risk that varies by how the loan is written and by Ohio law. A real estate attorney in Ohio needs to look at the actual loan documents before this goes any further.

The thing worth sitting with is that a long-term tenant with eight years of on-time payments is a meaningful asset in a seller-financed deal. Buyer vetting is one of the hardest parts of carrying a note, and you have eight years of data already.

What does the current balance on the loan look like relative to what the duplex would sell for?

The wrap is the part that almost never works the way people picture it. What actually happens is the lender finds out, accelerates, and now your tenant-buyer has no rate advantage and a closing crisis at the same time.

What type of loan does your aunt have sitting on it right now?

A subject-to or wrap buys her your rate but puts you on the hook if the lender calls the note; a clean seller carry at a negotiated rate costs her more but removes that exposure entirely.