However, HELOC allows it be in 2nd position and your 1st mortgage with 2.99% remains as first position.

We're assuming we'll remain at or near existing rents for the next few years, and planning on it if our current tenants stay. 

Laura  Last I heard, which was ~6 months ago, Idyllwild had ~25 STR permits remaining.  

Despite what a few have said, I've seen prices and demand here remain relatively steady thanks to proximity to SF Bay Area and Lake Tahoe.

when it comes to the investment side I have chosen to largely remain passive investing in REIT's or other publicly traded options.

Based on that I'd recommend: buying an SFR with as little money down possible (3.5% FHA or low conventional) using the remainder of the money to build a JADU and an ADU (specialized ADU lender for the remaining amount needed to build the units).

As long as there is a good thesis as to why the demand will remain high for shorter stay rentals  and most importantly (1) the use is allowed, (2) the use requirements are protected by in place permitting and licensing and (3) the permissions are transferable through a sale you will be on the right side of the supply/ demand equation.

Let's assume 8% Management fee, that's 100 less. 500 remaining.

The remaining were from $68K to about 87.

As a Spark investor, you are effectively paying double fees (Rise48's and Spark's) and then the cash flow coming from Rise48 is stripping 30% off for their carry, and of the remaining, Spark is stripping another 20% for their carry.