We would be paid an hourly rate and whatever we  spend/accumulate; the friend would get the cash rent until the differance is paid. 

If you budget the negative cash flow over the course of the hold period and have reserves set aside to fill the gap, there is no problem (assuming your investment meets your target appreciation rates - I understand there is no guarantee there). 

Even on a high interest rate environment  the market has been climbing.

Quote from @Devin James: Quote from @Andrew Syrios: “If you want to increase your success rate, double your failure rate.”- Former IBM CEO Thomas Watson This is my first time hearing this..

I've always found inventory rates (and how they've moved over time ie previous 1-2 years) to be the starting point for where a market's health is at.

.• Primary residence: A 2-bed, 2.5-bath townhouse purchased in 2020 with a 2.75% interest rate.

I don't know your specific market all that well but have you done some analysis on your ADR during the high season X your high season occupancy rate and determined whether taking lower priced slow season bookings vs letting your property stay vacant during the winter averages out in your favor?

Rising interest rates at the time created downward pressure on our ARV.

Even really skilled operators are left with no back up plan because they can't pivot to LTR in that market, can´t fetch the price they paid because the STR has lost some steam and was bought using nightly rate numbers etc.

If one or more units are vacant, the remaining units often don’t provide enough cash flow to cover expenses, making it harder to meet operating costs.Maintenance costs: A fourplex comes with four times the appliances, plumbing, HVAC systems, and other components to maintain, leading to significantly higher repair and maintenance expenses than single-family homes.If you want to see the detailed calculation, read this BP blog - More Units Doesn’t Mean More Money—Why a Single-Family Home Can Beat a Fourplex.Resale value: Multi-family properties have a limited buyer pool—mainly investors—who base their offers on CAP rates.