I've been in a very similar situation to you. It took me ages to find a decent tent heater. I thought I'd share some resources that helped me find mine:
I use a thermostatically controlled fan heater with two heat settings with maximum power consumption of 2kW.
I also have a butane only heater gas heater that was sold as suitable for indoor use that I used in the past with a draughty polyester tent without SIG in the living area when I was awake. However I do not use that anymore after switching to canvas with ZIP/SIG.
DK
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