Interest rate on a land contract feels like the part where I'd get in trouble

All three of those limits are real and they bind at different points.

The legal one comes first. Most states set a maximum rate for private lending, usury caps, though a handful including Nevada, Utah, South Dakota and New Hampshire let the parties agree to any rate in a written contract. Where a cap exists it differs by state, by whether the borrower is a person or an entity, and sometimes by the type of property. Some states are permissive, some are not, and exceeding the cap can cost you the interest or worse. There are also federal rules that can attach to seller financing of residential property, including ability to repay and loan originator requirements, and whether they reach you depends on how many parcels you sell on terms and whether the buyer intends to live there. That's a question for a lawyer licensed in the state you're selling in, and it's worth asking before your first note, not your tenth.

The practical limit is your buyer's payment. Most rural land buyers on terms are shopping the monthly number, not the rate. On your $22,000 at 9.5% over 10 years the payment is roughly $285. Push the rate to 15% and it's around $355 for the same balance. If your buyer pool tops out around $300 a month, the higher rate doesn't get you more money, it shrinks the pool and lengthens your marketing time.

The piece worth folding into your thinking now is that your return isn't only the rate. It's the markup on the parcel, the down payment, and the rate together. Plenty of operators keep the rate modest and take the margin in the price, because the price doesn't have a statutory cap and in most states the interest does.

The default and remedy language matters more than the rate and it's usually the shorter section. In some states a contract for deed with substantial equity paid in gets treated like a mortgage on default, meaning you foreclose rather than just cancel, and that's a court process with a timeline and legal fees. If your document says forfeiture in 30 days and the state says otherwise, the state wins.

One thing that surprised me from the improvement side: if your buyer builds a cabin or puts in a well while they're still paying, the equity in that parcel just went up and so did their incentive to fight any cancellation. A $22,000 balance with a $30,000 cabin sitting on it is a different problem than bare dirt.

@anchor the reason 18% isn't standard has less to do with tolerance than with what it does to your yield if the note dies. Underwrite the rate assuming a chunk of your notes stop paying in the first two years, because that's where they stop, and then look at whether the extra points ever showed up. On a $22,000 balance a buyer who quits at month 14 has paid you about $4,000 of which most is interest, and you now own a parcel that a previous owner has been mowing and maybe clearing. Higher rate, faster default, same resale. The rate is the least interesting variable on the page.