I do not recall there being a cost seg portion related to the STRs that make It better but I'm sure doing a standard cost set will also help for tax purposes depending on your goals. 

These 6 acres are a portion of 35+ acres we'd eventually like to purchase as well. 

Balancing both goals can be achieved through a hybrid approach, where a portion of funds is allocated towards paying down the primary residence while also saving or investing in another property.

He gets terms for his build portion and you have to put up actual funds.

You do need Form 4797 to calculate and report depreciation recapture and pay tax on this portion. 

. - Conventional purchase with 20% down using a HELOC: Current cashflow from the SF could cover a big portion of the HELOC payment, but this starts to feel risky from a cashflow and funds available for rehab.- Conventional and move there/rent out my primary: Certainly the best option, but a hard no from my S.O.'

There's a lot of value (even if a higher interest rate) to us finding a simple deal like this...We have great credit and solid income - I would think, for the right rate, there would be lenders out there jumping at the chance for these guaranteed returns - we'd even consider putting up the equity of the Texas home (and obviously the land itself) to secure some portion/all of the loan?

Is it possible to do this strategy if the “all cash” portion is a short-term loan from one of these “cash offer loan lenders” (e.g.

She agreed to refund a portion of our money since they never completed the onboarding process.

Yes, currently there are new builds going for 30k off, with builder buying down points, and covering a portion of closing costs.