I wondered if SF route might allow us to secure better terms at maybe a 4% interest rate for a few years on the funds not rolled into current $150k potentially assumable mortgage -- and in a few years we could roll into a traditional mortgage. 

I’m trying to work with a seller who deals in off-market properties, but they only accept cash or an “unrestricted line of credit” for payment.I have about $200k in liquid funds and want to invest it all, but I'm running into challenges with payment methods.

Build a portfolio by establishing a reserve fund, leveraging equity, and networking with investors.Good luck!

I considered syndications which invest in car washes, self storage, multi-units and real estate debt funds but to be more passive but I've heard mixed reviews about syndications. 

I suppose that requires a second (or third or fourth) opinion, but if he's right, where can I pull cash from to both complete our primary home, and refill the RE emergency fund I'm currently depleting?

Most areas that aren't A or B neighborhoods do not support new construction without some outside subsidy funding.

Or do you mean the investor is funding/financing the purchase?

I currently have 2 preapprovals from HMLs with two offers in process needing Gap balance funding asap.  

If they are not, you can roll over your 401k into a Self Directed IRA (SD IRA) and invest in real estate that way. 

If you have the funds back let it go