Heat slow to warm up

Early last year I swapped gen 4 motor into the 2010 Prius. Everything worked great, till I had a coolant leak and the coolant ran very low, not sure how long this happened for or how hot the motor got when it happened. Found the leak Refilled coolant and the car has been driving without engine power performance issues for about 6 to 8k miles. So no harm no foul there(maybe, still doesn’t feel nice).

when I first swapped the motor I remember the heat would warm up in about 5 to 6 minutes idling in the driveway. Well fast forward and since the beginning of the winter the heat takes sooooooo long to warm up, like it doesn’t get warm till I drove ten minutes to the highway and spent 5 minutes on the highway type of long. During the first few miles of each time driving the engine won’t stay idling when coming to a stop, and in addition the heat has a noticeable drop in temperature once youre off the highway.

im no expert, but I suspect the thermostat is stuck shut or is sticky and that is causing the motor to think it is nice and toasty so it stops the car from idling to warm up the rest of the coolant outside of the block?

gen 4 engine has about 52k miles on it. There’s no white smoke or anything crazy, oil is clean. No lights or error displays. Electronic air vents seem to switch fine from head to feet and defrost.

Thank you very much for your time.