I’ve heard some investors negotiate with their providers for extended coverage or switch to a landlord or vacant property insurance plan until the spec sells. 

Quote from @Melody Bohannon: Prior to my investment, I asked my advisor, Mark Zabilowicz,  multiple times about the holding period and was consistently assured that I could request the return after six months.

Shop LTV (or cLTV which is combined Loan To Value, basically your first mortgage and 2nd mortgage combined loan to value), rates, payment terms, time to close,  and draw period (how long you can go into and out of your heloc). 

Whereas with an STR, even with clear english detailing any issues up front, is often booked on a whim with an overload of information digested in a short period of time.

Most who make $500k don't have nearly enough dry powder on the sideline to scale a real estate business quickly enough without making huge personal sacrifices for an extended period of time.

Period!

Exterior Deck or PatioAdding an outdoor living space like a deck or patio extends the usable living area and enhances the home's appeal, especially in regions with warm weather.In addition to these improvements, understanding the local real estate market and focusing on upgrades that align with buyer preferences in that area is crucial for maximizing ARV.Noah, thank you for this great feedback.

I could put a down payment for a conventional (20-25%) 550k duplex, but I feel it could hurt my chances of buying as many duplexes in a 5 year period (for example) if I go conventional VS an FHA at 3.5%. 

If you're allowed to object and you're still within your objection period, you may want to review your title commitment for objectionable matters that may give you a right to terminate without penalty.

This resource covers FEDERAL fair housing law, but I think some states (especially tenant friendly ones) may extend protections further and may even eliminate some of the exemptions.