I recommend people doing a side gig on what they specialize in.I am an accountant and when I worked within large public accounting firms, I had a side gig of bieng an accountant.I had a salary with my employer so regardless of how much more hours I put with them, i would be paid the same. 

Another book that Ive heard recommended is "Die with Zero"

I would recommend keeping them, and gradually increasing rent on par with market increase + 1 to 2%.You could get a better qualified tenant, but they could be worse with money management, property respect, etc.

I am closing on my 1st rental (Duplex) next week & I am going to self-manage the property. Since I only have these 2 units for now I was thinking of using one of the free websites (Cozy, Tenant cloud, etc...). I ...

- Any lessons learned or best practices you’d recommend for someone leaning toward this approach?

Years ago we didn't require it and just recommended it to the tenant, but it's so cheap and gives our insurer something/someone to abrogate against should we have to make a claim that we don't allow it to be optional anymore. 

Personal recommendations don't always work.

I'd recommend you sell the place and put your money in a good mutual fund.

I've seen the Speed Queen SF series recommended in a few places.

Then, I would recommend connecting with STR specific realtors in a given market.