Step 1 is to get an estimate of the amount of tax deductions that may be available through Cost Seg.

Second, you didn't mention how long they estimated it would take.  

Get fee sheets or loan estimates from whichever 2-3 lenders you're considering and make an apples to apples comparison.

I am working with the city for approval and have a cost estimate from someone I know of 250k to build another duplex on the property and eventually multiple duplex.

As long as you're financing it as an investment property you should be able to.You might have a hard time getting a lender to fund if you don't have rental rates/estimates to provide to the lender.

I did check airdna, mashvisor, str mgt company estimates, etc, prior to going str and I think I was pretty realistic.

What is the estimated cost of repairs? 

For example, divorce buyout calculators can help you estimate how to fairly split equity.

I have an opportunity to construct a mixed-use building with 42 residential units (each around 600 sq. ft. with 1+ bedrooms) and 2 commercial spaces, totaling approximately 42,000 sq. ft.We have a solid understanding of constructing smaller projects, such as 2-4 unit buildings, but this larger scale is new to us.

Make sure you give them a review on the platforms and if you want, you can report them to whatever accreditation board they have, but that may not result in anything.And get a second, independent estimate on repairs.