How do I remove a stuck tap bonnet to change the cartridge?
You might try loosening the adjustment ring, the brass ring with two slots for a spanner wrench that's inside the bonnet of many single-handle faucets.

Those spanners often come with faucet repair kits, along with springs, O-rings and other parts, but could be easily fabricated with some heavy-gauge sheet metal and two bolts with nuts; file the end of the bolts to fit the slots.
That said, the adjustment ring could be frozen as badly as the bonnet. Recently, I had to replace a similar faucet; the bonnet was stuck so tightly that on turning it with a large strap wrench, a rusted bolt holding the faucet to the sink broke! In that case, it was not a loss, because I had planned already on replacing the ancient faucet.