If you're looking for more active strategies like purchasing your own real estate let me know and I'm happy to send updated recommendations based on what your situation is and what your goals are :)

Did I just not do the math right on my purchase?

In order to defer all tax you must purchase at least as much real estate as you sold. 

I also am very interested in purchasing more STRs, and this is a really low risk way to see if I'm up for it (we have a 2.9% interest rate on our mortgage).My biggest concern is an ongoing plumbing issue we have had.

You get a Capitalization Rate (CAP Rate), which equals your annualized return by dividing the Net Operating Income (you had gross in your narrative) by the purchase price.

I'm looking for advice on whether I should purchase my father-in-law's 6 unit apartment in Burien, WA (10 minutes south of Seattle) as my first deal, or perhaps help find him a seller that would pay more than I could and is a couple years ahead of me in REI. 

Please, no solicitations.Use the renovation/acquisition loan products that require 10% down on the purchase and you should be able to get a few going at a time.

We found an opportunity to purchase a huge SFR with two kitchens located next to a commercial zone which was previously used as a sober living facility.

A lot of it has to do with location and time.If they are local owners looking at 1-2 LTR properties, I'll assist with identifying cash flowing/near cash flowing properties, assist them in the purchase as their buyers agent and assist with tenant procurement, listing, etc. but they can typically handle day to day management from there.Furnished properties, whether that be MTR or STR, are a lot more involved, as I'm sure you're aware of!

@Brian DolbeareI have never purchased a property with no off-street parking.