Both of the things you named are the right things to ask for, and the partial release is the one that matters more.
An extension option is straightforward to draft. Something like two twelve month extensions, exercisable if you're current, for a fee of half a point to a point each, sometimes with a small rate step-up. Sellers who want the interest stream often take it. Sellers who are carrying only because they couldn't get a cash buyer sometimes won't, and then you know the real term is five years.
The partial release clause needs specifics or it's decorative. It should say how the parcel can be divided, what release price per acre applies, how much of the sale proceeds go to principal, and that the released portion keeps legal access to a public road. A release that leaves the remainder landlocked destroys your own collateral. Get the release acres drawn on an exhibit, not described in a sentence.
On the refinance assumption at year five: raw land lending with no income is a thin market. Lenders who do it generally want a large down payment and a short amortization, and pricing moves, so treat a future refi as a maybe and confirm any current terms in writing with a lender before you rely on them.
The structural point you haven't raised is which instrument secures the debt. A note with a deed of trust or mortgage gives you a foreclosure process with equity protection. A contract for deed or land contract, where the seller keeps title until you pay in full, can allow forfeiture with far less protection for you in some states, and the rules differ by state. Which one you're signing changes what happens if year five goes badly, so have a real estate attorney in your state read it before you send money.
