If you're just trying to buy investment property doors as fast as possible without a liquid cash position you might want to try to focus on value-add to create some cash flow or BRRR opportunity for your next deal. 

With 20% down, mortgage, HOA, etc would be sitting right around $2500-2700/month range.

Not saying it needs to look like a magazine cover, but make them appealing to potential buyers if possible.

There are some who will not own a car (it is possible), but a large majority will have cars and more than one per unit without parking supplied by the builders.  

I am hoping to find something with 2 - 6 rental units, and I would like to be in the $300-$500k range.

HUGE time saver.Individual property accounting - keep track of how profitable (or not) each individual property (or unit) is.App for tenants - so they can look at their balance, make payments, report maintenance issues.Rental applications and tenant screening - Accept online rental applications and then run credit, criminal, eviction checks on them easily.Market your vacancies - getting a vacant unit filled with a good tenant as quick as possible is critical, and good software will help you market those vacancies. 

If there is enough equity at the 6 month mark you could do it then but there may not be much of a tangible benefit unless there is a PITI savings.If you are able to do some renovation in the first (6) months it might be possible to do it on the 6th month mark based on a higher renovation ARV.If you have any specific questions feel free to message me or reach out I am always happy to help and talk REI.

Ah, thanks for clarifying the 1031 rule..yep this was my personal residence.Goals wise, I'm just trying to get my money to work for me as efficiently as possible with a moderate amount of risk.

If it's possible, would I need to submit 1 or 2 short-term rental registrations?

You can always start off by house hacking meaning you live in one UNIT or ROOM and renting out the other vacant units/rooms to offset your monthly mortgage payment.There are several ways to run and make sure you are maximizing your rental income while keeping your living expense as low as possible.