PCB low power self heating component separation
Here is an approach to thermal isolation while maintaining some mechanical strength -(the chip is an ovenized buried zener voltage reference-  it would have a urethane foam insulator over it in actual operation): As well as the wide routed slots, notice the thinner ones to the left and right and the slots to the top and bottom.

The copper, even 34\$\mu\$m copper, will conduct a lot of the heat, so narrower and longer traces are called for, as well as limiting the amount of laminate (thermal conductivity is proportional to cross sectional area divided by length).
The thermal conductivity of PCB laminate is anisotropic and is much higher parallel to the surface than through the PCB due to the direction of the weave in the fiberglass cloth. For that reason, rotating the pattern 45° might make a noticeable difference.
But, personally, I would V-groove or mouse-bite the PCB with a thin soldered or socketed jumper over it and then snap it off and assemble it with insulation between the two if accuracy is that important.
Or you could consider just faking it and compensate for the error due to the other circuitry either with a second sensor or a thermal model. That would make an interesting project to burn some time if you're not busy.
P.S. Sometimes you can make use of power dissipation by mounting the hot parts towards the top of a ventilated enclosure so that air at room temperature is drawn in at the bottom, over the sensor. I suspect your power levels are way too low for that to be a useful effect in this case, though.