In our other car, a 2024 Aygo X, we have this problem. Not on the Crosstar as it is 2021 car.
The Toyota speed bongs were driving me mad, found it way too distracting. And I stick to speed limits.
But in situation where you start to speed up just before the end of the 30 zone (say) the bloody bongs would start! I had to totally clear the 30 zone before speeding up to stop the bongs happening. As I was taught to speed up into a higher speed zone slightly before the new speed limit sign by my driving instructor years ago, that's what ive always done, without taking the piss. The rest of the time, I am absolutely within the speed limits. Also too, had the other issue mentioned when I set my cruise control to 31 to take account of the speedo underread, and the bongs would go off. Easily remedied.
However I now go the route of turning off the Road Sign Speed Recognition system on every drive, which take me seconds, and stops the bonging.
I really hope this issue doesn't get worse with increasing legislation. Although I am totally in favour of measures to warn speeding drivers generally.