Honestly, it’s not as complicated as you are making it out to be.
First, your purchase price doesn’t matter in the “sell-vs-rent” calculation. That’s a sunk cost - what matters is what you could get now for selling vs what you could get if you rented it out. Factor in what you anticipate it might be worth in future years if you like, but realize the further out you extrapolate, the less confidence you will have with those numbers.
Selling: you could likely get $900k minus ~6%. Again, it doesn’t matter what you bought it for. You could have inherited the home and it wouldn’t change the sell-vs-rent calculation, or you could have paid $1.5MM and the answer would still be the same.
Renting: My rule of thumb is if you can’t get 1% of the sale price renting per month, walk away. Great rentals go for more (the golden unicorn is 2%, though that’s impossible to find in many markets). Some will go down as low as 0.5%, but that’s pretty razor-thin. If you are looking at $3,600 you will be taking a bath at 0.4%.
Consider that - long term average - roughly 50% of your rent goes towards maintenance, vacancies and insurance. That leaves you with $1,800/mo.
I’d say where you “messed up” (your words) is not considering opportunity cost from not selling, using overly optimistic assumptions about your expenses to rent (again, factor 50% long-term) and then price-anchoring yourself in the past. If $3.6k/mo would “completely cover your PITI (today), what would $1.8k/mo do? Some quick calculations suggest your PI is around $2,500/mo (assuming 20% down on $725k @ 3%). Adding in taxes and insurance and I'm guessing you are well over $3000k/mo for your total mortgage payment. That doesn't leave hardly anything to cashflow your property. Now consider the opportunity cost of selling - you’d have six-figures in your pocket after paying off your PITI. That, over the next few decades will compound, whereas you are likely not even going to cash-flow renting at $3.6k/mo