Are you running 'X bars' in your roll cage? If you are...is it permissible in the class you're in to remove the side impact bars in the OEM doors as there is huge weight in these.
As mentioned... there is huge weight in the OEM heater system too. The below is only 1.15kg... so big saving to be had here..
https://www.demon-tweeks.com/eu/demon-tweeks-lightweight-heater-tsdlwh/
Remove as much as possible from within the dash too... unnecessary plastic trim, plastic pipework, loom etc. It all adds up. Is the sound-deadening removed?
Next heavy thing you could look at is sub frames... I've seen some that were heavily drilled to remove weight but also seam welded for strength. If your wallet will allow... tubular sub frames could half the combined 20kg weight that the OEM ones come in at.