However she regularly uses my maintenance staff on my unit and occasionally on other units (she recently used them for a few days to furnish a new STR including transporting the furniture - I also believe she under paid them as she paid their usual rate but they provided a trailer and vehicle to get the furniture).    

Well positioned, quality, strategic investment real estate BEATS S&P, beats "cheap cash-flow" properties, beats most every investment vehicle in existence. 

Take the excess money you would pay in principal & put it into a vehicle that would yield you 7.5% or greater...If you find another RE deal at a 10% cap rate , interest only can be a good thing, it frees up more cashflow to reinvest in better ROI vehicles. 

I don’t know the vehicle-per-day traffic counts (in Texas I’d check TX DOT, you might check Vermont’s equivalent), but there could be potential for more density or alternative low-impact commercial uses that don’t rely on heavy traffic.

There are many vehicles to get that equity and use it to make additional investments with out having to sell.  

The new funds are being invested in a separate preferred equity vehicle, but the benefits would flow to the entire original investment.My key question: Does using retirement funds (solo 401K or Roth SDIRA) to support a deal where my personal trust also holds equity create a prohibited transaction under IRS rules, even if the entities are technically investing in different vehicles?

If you use that at a vehicle over the 401k is that a better option? 

Top Zip Codes for ROI – June 2025:28213 – Mecklenburg County- ROI: 7% | Vacancy: 7% Anchored by UNC Charlotte and the Blue Line Light Rail, this area continues to attract students, professionals, and long-term renters.

If the unit is rented, the tenant is responsible for damage caused by a 3rd party (at least per my lease).If OP leased a vehicle and a 3rd party damaged it (even if it is the police), does the OP believe this is the responsibility of the company that leased the car to the OP?

For section 8:  in some states you have to accept it so your PM should know the law if you have to accept you just try to find a better qualified tenant or set standards that insure your section 8 tenant is good. credit score, max occupancy, max vehicles.