other lenders have been too inflexible with rigid standards

Nearly every single property I have came with some quirk that turned off other buyers, but that was largely inconsequential to my plans (for instance, an oddly-placed staircase, ugly old carpet, a location that was on a somewhat busier street, etc.)...if I had absolutely strict, inflexible criteria, I might not have looked at those properties.

Banks definitely offer the best rates, but the downside is spelled out exactly in your experience...deals fall through due to stringent and inflexible underwriting

This builder is located outside of Orlando,FL and usually has 3 town homes in inventory, so demand is not too high.They're offering $15k in flex costs (used towards closing or design studio) if I go in house financing.

I am not opposed to keeping a W2, but with the income expectations and the change in flexibility for time and location, a transition away from W2 may need to happen. 

These will say either "Yes" or "No", and can be inflexible if your needs don't quite fit the box.

My goal is to buy 1 property by the end of 2021 with cash flow of $150+ per unit and a cash-on-cash return of 15% (although I've been cautioned not to become too inflexible with those numbers).

They should have very good troubleshooting skills, both for talking a Tenant through a problem that avoids any cost to Owner, and so they understand what contractors are actually talking about on projects.You do also need to realize, THEIR systems are just that, they may be inflexible in some, but not all, areas.

I am also interested in flex spaces and storage facilities.

But there are some very inflexible procedural rules and reinvestment requirements.