I solved the issue by linking my lights to a switch that I turn on when needed. It has worked very well so far, with no false detection at all!

plz let us know how it goes after sufficient testi g

Hmmmmm that's not a bad idea, I'll have to see how to do that, though. I don't want cables restricting me.

If your fine with zigbee, I could see having one of these aqara door sensors in the cushion of the seat, triggering to closed when you sit on it: https://cloudfree.shop/product/aqara-door-and-window-sensors...

Seems like it'd be finicky though.

Zigbee is great, but my chair has a mesh :/ I'll see if I can print something below, so that the sensor is activated, but yes, it does seem that it would be pretty finicky...

You can buy the sensors they use in car seats for like $10 https://a.co/d/0dibAFxc

Ceiling sensor is another option.

For now I've put a mmWave sensor under my desk, which works fairly OK. It's not perfect, but definitely good enough.