Its a good thing they profited, because projects can go south sometimes and then that really hurts.  

The hard part on every fix and flip is to find a property with enough meet on the bone to make a good profit.

I think you run a real risk of your second rental eating up the profits you're enjoying on the first one.

During 2020 to 2022 I was fighting to keep my business profitable as materials and labor came in short supply and eventually led to the inflation we're seeing now.

I have 2 options:- finance the construction pre-selling the apt, and take my profits (expected to be around 700K pre-tax); or- finance the construction with a 6-7%/year loan (collateral = my house), keep the 10 units, and once the building is completed in 24 months, rent them for circa 180K/ year gross.

And I bought 3 other properties with a different deal structure, but also profitable. 

At over 4000 sq ft, I'm surprised you won't be able to list for a much higher number to net a decent profit, but without knowing more specifics on the house and location, I am speculating here. 

The profit was used to buy a higher quality SFH in the same area.

I want to profit off the interest rate and perhaps offer 6% rate on what is left on the mortgage.

I've Had 1 rental that I sold for a good profit recently.