Light bulbs are blinking when controlling powerful heater with PWM via SSR
Add noise or a signal large enough to thermistor feedback . Instead of PWM, rely on ZCS to skip cycles and get proportional feedback with cycle skipping modulated by noise and ZCS for so sinusoid step current which ceiling lights may withstand.

Choose noise level to match your proportional range 1 to 10 deg or so depending on dT (‘C) in 300 ms

Line frequency noise may or may not be ok.  In the late 70’s I had a waterbed heater.  I designed OpAmpwith some AC noise so the relay switch so that it would skip cycles every 10s to 10 minute quietly with high derating  to last 100k ~1M cycles to regulate within 0.1’C 

Dual thermistor is better due to sense errors near heater.

added

If I assume you have 230Vac at 1.5kW or 6.5A load causing lights to flicker, I wonder why? are they LED or FL tubes with sensitive to line regulation errors?  Then certainly higher PWM will work better with a different power supply , e.g. http://www.ti.com/tool/TIDA-00779

A more ideal solution.

Active Power Factor Correction, for noise immunity to interference to others and acceptable conducted &amp; radiated noise, easily regulated and noise compatible. (good EMC design)