My purchase price was $187,000, meaning I SHOULD have about 50k in equity

I have roughly $120k in equity currently

Holding the property for 12+ months, could result in much lower tax burden, or (2) holding your investments and using the built-in equity to finance additional deals.

I even managed, within the positive cashflowrange, to take out approximately 40.000$ of the appreciation in equity.

I wouldn't say these are cash cows, but today I sit on approximately $400k in equity across all properties, and generate roughly $1100 in positive cash flow monthly (before maintenance).I found out over the past 12 years that I actually enjoy being a landlord.  

I have about 15,000 in cash and have about 35,000 to 40,000 in equity in my rental property.

If it were me in the same position as you, I'd keep it as a rental, $10k in equity isn't enough t make me want to sell, I couldn't reinvest that in anything I'd get too excited about without throwing in some more money. 

In our example after 10 years you will only owe $92,644 on the building IF the property never appreciated one cent and it was still worth $150k in 10 years you would have $57,356 in equity.

If I need to put money down, I will strike a JVP and buy my partner out after 6 months with the refiIs this a bad deal since there's no built-in equity?

So you should expect at least a 10.5% return on your investment PLUS about $5,000 in equity paid down each year PLUS any appreciation gained.