Engine water temp very low at an idle. Takes long time to get to 200- deg. When driving. Changed thermostats not much change. Heater not hot . Comes and goes on heat veryation on the cool side of normal.
Okay, You stated that you “changed thermostats” so I am assuming that you changed both the front and rear thermostats with oem rated replacements.
How long is a “long time” to get to 200 degrees?
What is the ambient temperature where you are at?
With the temperature sensor being mounted high in the engine and your statement “engine water temp very low at an idle” you may have air trapped in the system.
As far as the heater not being hot that could be a problem with the blend/temperature door actuator, restricted heater core or possibly a weak water pump.