Add another $20 a month for that.Add in HOA fees ($300 a month; shown in your link), Gas, electricity, high-speed internet, water, trash and snow removal. 

Does the effectiveness of each vary by location?

Over my 44 years I learned how to maximize my profits and deal with the frustration very effectively.

The free marketing is not effective in a competitive market neither is the marketing strategy.

Our membership dues are under $200 per year, so I'd say it's well worth the money to have some representation.That being said, from what I've heard we are more organized and effective (possibly out of necessity) than many STRA's out there.

Demolition and Removal: Outline areas to be gutted or removed (walls, flooring, fixtures, etc.). 

If you ask the manager to justify a fee and he starts hemming and hawing, move on or require them to remove the fee.

I don't have experience with STESSA, but have managed my own small portfolio with QuickBooks Online pretty effectively.

You can't hold them accountable to the terms if they aren't on the lease.I would remove all of them from the rental and start fresh.

My primary concern with two separate leases is that it would be extremely challenging for you to remove a tenant via eviction for any reason because technically it is one unit.My recommendation is to provide one master leases signed by both groups of occupants.