I am seeking guidance on how I should address this turnover issue. 

Seek legal advice, consult a property manager, and obtain multiple repair quotes for a well-informed decision.

Jennifer, I'm a real estate investor and seeking Mastermind partners to form JV's/Syndications to build a portfolio of multifamily properties.

This ADU is an improvement to your property which is just increasing your basis in the property.

Purchasing with a mortgage can increase this return to 12-18% pretty easily.

When the property is refinanced a year later, after it has been owner occupied for the required amount of time, will the rate then increase to that of an investment property?

Increase cash reserves for larger purchases.

A higher credit score increases your chances of obtaining a favorable loan.Investigate several lending choices, such as conventional mortgages, FHA 203(k) loans, and other rehab loans intended for home upgrades.

Seeking an early release for family reasons, with 60 days notice. 

Once you have this team in place, you should be able to invest in any market confidently.As for picking a specific market - I would go after one with an increasing job and population growth.