I had a tenant give the proper 30 day notice that they will be terminating the lease because they will be purchasing a home.

The source is likely from the bathroom not being properly vented while the shower is on.

I also own properties in landlord-friendly states where there's no rent control, so I don't need to pay someone to leave in order to increase the rent.To address the other points in your post:This is why having a proper purchase and sale agreement is so important.

(They have the deferred maintenance you'd expect after a fixed income senior lived in them for several years, plus smokers, pets)So how do I turn 5 straggling houses into a proper real estate portfolio and well-managed rental business? 

I would make sure your property is properly marketed and that your rents are in line with the comps.

Primarily when downsizing they question your occupancy intent (Claiming primary residence on a fixer upper for favorable LTV terms, with intent to convert to an investment in the short term) but everything can be worked out with proper documentation.

A building is expected to have a functional roof for example and it will not give you value vs other properties who also have a functional roof. 

Take ownership of your mistake and learn to do the proper due diligence recommended above😊

I wrote a BP blog about how to screen tenants properly here. 

But then again, I've heard that as long as you keep everything up to par with the llc's basics  (separate bank acct, current llc status w state,  properly funded,  no co-mingling, etc) it holds up in court even if single member.