As you see, Unreal 4.26 have a new TAA system, called Gen5 TAA, he can improve things by a lot (with a performance cost).
Unfortunally, and yes, Unreal Engine doesn’t have the best TAA in the world.
I don’t think there is magic solution at this day.
After you don’t gonna like this answer but… if having a good TAA is a need for your project, and unity fill this need perfectly, it can be a good idea to go on Unity for your project. I know this is far from the best answer but there is some smart advice beyond this as “Use the tool who fit the best to the job”.