OK here's my new plan, please let me know if you see any flaws.
I'll order two hydrofarms rated for 1,000 watt and run the heater at 600 watts.
I will plug Tstat #1 into the wall and set it at 10 degrees above the desired room temperature.
I will plug tstat #2 into tstat number one and set it at the desired temperature.
I will plug the oil radiant heater into tstat #2.
If tstat number 2 fails open, tstat 1 should still cut the power at temperature that are higher than desired but pose no short term risk.
It seems like two hydrofarms would still be cheaper than one ranco and would be safer because the odds of two tstats failing open at the same time would be low?