He verbally proposed an idea that I move in and pay rent on the whole property while collecting the income from the ADU.

(I'd still be putting aside the 15% as if it were fully rented because of Murphy's law) I'd have to live there a year, then move (another duplex?)

I am actually moving out of my house this weekend, then I was planning to do a long term rental but maybe STR would be more cash flow?

Some loan options are:If you're looking to buy a property that's not move-in ready: You can get a fix and flip loan for an investment property with up to 90% of the purchase price and 100% of the rehab budget with up to 75% of the ARV.

After moving out I was able to cashflow ~$1,000/month but ended up refinancing and getting all my money out of it, still cash-flowing $450/month, and using the money to buy me another rental property.

I plan on using a property management company in order to keep the property in order until I move (eventually I want to bring property managers under my own business entity to focus only on my properties). 

After moving out I was able to cashflow ~$1,000 a month but ended up refinancing and getting all my money out of it.

Having good processes in place is particularly important for tenant screening, move-ins and move-outs and the handling of security deposits, but I don't know how you can have processes to guard against criminals and unfair government policies like those in place in some areas during the pandemic.

Does it make sense to invest in say a 350k property that will cost me 200-300 a month to operate even after I move out if its in a good neighborhood? 

There are more moving parts between cleanings, reviews, keeping inventory of a furnished place, bookings, new people every few days, people not understanding the difference between an airbnb and hotel etc.