coronaviruses do not mutate as quickly as influenza so it’s a bit of a different situation. The 3 dose mRNA + booster is all the same exact vaccine. That being said, maybe we will need annual boosters which we get with a combined flu shot.
Yes, currently it is. But that may change at any time.
So we already have thousands of mutations. But changes we call variants happen much slower.
Still, the vaccins are based on the 2019 variant.