Post #2 asked for a pic of your crossover cooling system.

Some styles you can’t just put in a thermostat because it will block the whole systems water until it opens, and that is no good.

I will get the pic posted hopefully soon the boat it at my lake house I just need to get into the shop to take the pic hopefully very soon!

My water temps never move, it`s not a problem.
You do need to know what your oil temps are. Takes me a while a while to get my oil temp up too (15+ minutes for at least 120*)
I installed a 120V 500W oil pan heater that I turn on at the dock when we wake up but it still took over an hour to heat them 14quarts. to 100*
So then I installed a second immersion heater in the pan and that must have been too much amp draw, I burned out both the heaters .
back to square one lol

anyway that`s just over kill but you need oil temp don`t worry about the water temp.

On my last boat I installed an external oil thermostat.

EDIT :
https://www.summitracing.com/parts/ctr-22-480


Made by Canton if I recall