I would just start out by meeting with bankers, telling them your goals and mentioning some of the ideas you had(FHA vs conventional) and ask for their advice.

If you go with a conventional loan for the cash out refinance, seasoning periods are longer which draws out the process. 

What happens if you buy the duplex with a conventional loan with the purpose to build an ADU to move in shortly afterwards?

FHA or Conventional loan?

Will riverside continue to build keeping inventory steady and will people need to commute in traffic to get to work? 

@Rick Albert   I may get a lot of push back from the team but I split my utilities on my ADU the conventional way....each dwelling has its own meter for water, electric and gas. 

Right now, we’re seeing a slight increase in inventory and some corrections in rents, which could create opportunities—but achieving solid cash flow on a condo can still be challenging.Getting pre-approved is a great first step to narrow down your options and focus on locations that make sense for your goals.

As an investor, you'll need to wait until this period expires before your offer will be considered.Additionally, Freddie Mac may require proof of funds or a pre-approval letter with your offer, and they often prefer cash offers or conventional financing.

. - you have dozens of local agents who can market the property for you and bring you a buyer- you are listing in a desirable area with low-inventory - you are not in a rush to sell

Also, make sure there aren't additional restrictions on the property condition, compared to a conventional loan.