Rad Flush - antifreeze question

Got into it this morning and opened the rad cap, no room to add any more coolant. Let it warm up. And put it on a steeper incline to help with air bubbles. Turned the heater off and steam started coming off the rad and temp gauge shot up, so turned heater back on and this stopped steam and lowered temp gauge back to normal. Could have been a coincidence so waiting for it to cool again and will test with no heater. Took it around the block also and engine didnt die on me this time. Hoses are all hot but quite hard to squish. Dont know how pressurized they should be but concerned that they are under a lot of pressure.

Still quite concerned it's the thermostat or water pump not opening fully or slow flow (I should have tested the bloody tstat before installing.

I may just take it to the shop and have them diagnose it.