Why make calculation complicated.
You buy 850K (after adding in renovation 50K) and sell also 850K, there is no capital appreciation at all.
So you return only rely on your rental 3.5K

3.5K/850k = 4.1%.
And your mortgages interest rate is likely to be 4.xx%.

So most rental money made is going to pay the interest.
And the 3.5K is not yet deduct expenses related, like maintenance fee, misc assessment, quit rent etc.

You don't need complex mathematics to know it won't be good return.

Sorry for mistake in calculation
3.5K x 12 / 850K = 4.9% yield

Rental yield indeed superb for current market, as a lot of residential yield below 2 to 3% only.
But loan interest also 4.xx%.

Property is about capital appreciation, and where the big money is.