One UI 3.5 will be called 3.1.1 and will launch with august unpacked event(z fold 3) then one UI 4 will be based on android 12. Samsung will start testing oneUI 4 for S series from next month.
Not yet, but i will keep updating. Also android 12 is alleged to have 20% improvement in smoothness so that can be expected. And for now the videos I have seen about android 12 beta 2.1 looks promising. Animations are nice, fluidity is definitely there. Let's see how android world gonna implement it in final version.