I think you could legitimately use your SDIRA to buy land and have someone plant trees and have them harvested every 10 years, you could invest in a syndication where someone else manages the properties, or you might be able to get away with buying a self-storage and getting paid a reasonable salary to run it. 

In the progress of weeding/ harvesting equity from duplexes which are good- but not great cash flowers.

In order to find low yield investors it will take some DIY elbow-grease whereby you would need to get in there and cultivate and harvest your own SDIRA investor-leads outside of pre-programmed, SDIRA meetups. 

The Mills they would never do that otherwise they get black balled and no one will deal with them.. but johnny lunch bucket logger... they would cut you out in a heart beat. however most owners would honor the sanctity of contract even though there was no laws or rules pertaining to timber harvest contracts.... 

Plus it makes it easier for them to decide the crops aren’t worth harvesting as it will save them so much rent, fuel, time, and they can just harvest on their own land instead.

They improve the land by harvesting trees, which in it self can make you a little money or recoup some cost.

The strategy you describe is often called “harvesting” your portfolio.  

He would look for wooded land with trees that when harvested would bring top dollar.

These all help and allow you to harvest equity for further leveraged  growth. 

Frank  The 2nd note or taking HELOC's idea on your other rental or current primary residence (duplex) is a partial solution but I doubt there is enough "equity," to harvest that would allow you to complete your entire construction of this new raw lot SFR + ADU's or units by it self, solely.