It’s worth about $2.5m and I still have to finish the yard and would like to eventually build an ADU.  

And if they were renting, I gotta think they'd be looking to buy a home eventually and they'd know they'd need to pay less for rent so they could save money to buy.One thing you could consider is possibly looking at furnishing it and making it an STR or MTR. 

My plan was to "self-perform" a land contract by filling out a purchase and sales agreement for a land contract that specifies I will take over the mortgage payments, get authorized on their mortgage, and be authorized essentially to fix up the property and eventually sell it on the MLS since the land contract will give me that control/equitable interest.

However I do plan to move out of state eventually.

The marketplace is always evolving, so may eventually see more listings where sellers only set their listing agent commission for marketing the property and negotiating on their behalf, and not offer any buyer agency fee up front, and just settle on that in the Offer the buyer agent brings them.

Based on my own experience, when I found myself spending thousands on rentals for construction projects, I eventually bought a Bobcat and it paid off within two years.

Anyway, he eventually won me over and I took achance on him.

An arrest was eventually made a few years later: https://www.fdle.state.fl.us/News/2022/May/FDLE-makes-arrest...   

Eventually, we found ourselves being the first call when they were ready to make a move.We didn’t rush them, we didn’t push for the sale, we stayed patient and persistent—and when the time was right, those long-standing relationships turned into deals that no one else in the market knew about.This approach not only led to deals but also built our reputation.

I would explain that to them that if they don't stop being obstructionist that they stand to end up with a lien on their house as a result and eventually lose that house.