Why does my smart toilet keep tripping the GFCI every time the seat heats up?

The strip is a big part of your problem. A GFCI trips on current leaking to ground, not on too much total draw, so it's not that the heater is 'too much' for the outlet. But every device plugged into that circuit adds its own tiny bit of leakage, and a dehumidifier compressor is notorious for it. Stack the dehumidifier plus a nightlight plus a heated seat all through one GFCI and you can creep right up to the trip point. You're sitting right at the edge, and any little extra nudge pushes it over. Unplug the seat from the strip and give it its own wall outlet and you may find the problem mostly goes away.

The funny part is the 'only when I sit on it' thing actually makes sense. Your body is sitting on a heater element and touching a grounded fixture, and that adds a little leakage path that isn't there when the seat's empty. Bigger person, more of it. Same reason my heated mattress pad only tripped the breaker when my wife and I were both in bed, never for one of us. I borrowed a plug-in GFCI tester and confirmed the outlet itself held fine with nothing else on the circuit, which told me it was cumulative leakage and not a bad receptacle. Moving the pad to its own outlet ended it for good.

The outlet itself can be the culprit too. I chased a similar nuisance trip on our heated seat for months, swapping cords and unplugging everythign else, til it finally clicked that the GFCI receptacle was over twenty years old and had gotten touchy from years of steam and cleaning spray. Swapped it for a new one and the phantom trips stopped that same week. They do wear out and get more sensitive over time, which lines up with how yours seemed fine at first and then got worse. If moving it off the strip doesn't fully fix it, I'd try a fresh receptacle before blaming the seat.