I wanted very fine adjust ability of ramp rate on a high voltage heater (400W)
Resistive 400W @ 230VAC means about 1.7A < 2A current, so easy task for triac or... AC mosfets switch with mains current & voltage sensing.
Why AC mosfets switch not triac... because of AC mosfets switch can be turned ON/OFF when I want, while triac or thyristor when current drops to some level.
So It looks like very easy task to limit this output power to very low levels when you have mains current & voltage sensing while you can estimate in MCU power consumed for heating and you are able to triger switching ON this AC mosfets switch at the end of 10ms half perdiod in the case of 50Hz 230VAC mains, so I do not think that any PWM is needed to controll this heater, but simply monitor mains current & voltage and input only required amount of power by adjusting phase when in each 10ms period AC mosfets switch is turned ON/OFF.
I sucessfully used before custom light dimmer (but classic with diac only higher current rating of used BTA triac) to limit heat power of 2kW 230VAC 50Hz oil heater, but in recent spot welder project I will test also this AC mosfets switch with mains monitoring aproach too, while it does not require any rectifiers and by putting many mosfets in pararell I can get better RDSON than with triac I hope.
I think you could simply use:
mains-fuse-filter-current&volatge_sensor-ac_switch-rectifier-capacitors-heater-temperature_sensor.
Whatever you do still PID controll paremeters or other fancy methods including determined heat transfers models might be needed fro such precise temperature control, but it looks like PWM after rectifieed DC is not needed, while we can use AC phase shifting and predict/set needed amount of input power to heater simply by adjusting delays time.
The main problem might be to know how much energy is needed to do not overheat this thing in changing ambient temperature, I guess