> RC timing, capacitor C changes with rising temp

The only option is to use a thermistor (ptc) together with the original
timing resistor (in series).

I don't know how accurate your timing must be but using an 85 C cap at 80
C is not such a good idea imho. A 105 C rated one is a better choice imho.
This will also likely be more accurate and have less problems with
temperature changes (different electrolyte etc).

BTW does the mains voltage not affect your design ? It can vary by +5/-5%
usually, f.ex. due to increased load in a longer house feed line.

hope this helps,

Peter