I have used two different boilers, a Taylor 450 OWB and now a Econoburn 200.
Many things to tell you and too much to type it all. If you pm me I would be happy to talk to you.
It takes a lot of btus to heat a pool. I have a 15,000 gallon in ground and it takes roughly 120,000 btus to go up 1 degree.
I would;
Fill the pool and let the sun help you
Set up the heat exchanger with bypass (you can not run your pool through 3/4" pipe)
I have a stainless shell and tube exchanger, keep your chlorine below 3 and your ph 7.5 and it will help the exchanger. The bypass will let you isolate when you shock.
Gg