Su-57 is supposed to be able to use lower observable coatings which can be applied in case of wartime but doesn't use those by default.
Only cheaper to maintain low observable materials are used by default.
Why the F-35 doesn't use a similar scheme is beyond me.
With regards to the engine, GE and Rolls Royce worked on it for the ATF competition and also in later programs. The prototype flew in the ATF competition. One later program investigated its use in a 6th generation application not long ago. I doubt it would cost that much in R&D to bring it to production.
F-35's coating is far easier to apply than that of B-2 and F-22. It is more resistant to effects of weather as well.
I think the new 4.5th gen doesn't make sense unless the U.S. is hellbent on maintaining a fleet size of over 2000 fighter aircraft. Researching and developing a completely new aircraft that would be obsolete in two decades doesn't make much sense once the R&D costs are factored in. One possibility is that the air force is still toying with what the true definition of a sixth generation fighter is. Keep in mind that before the Raptor made stealth a prerequisite, Russian and European designs for their next generation fighter aircraft focused on ultramaneuverability instead of stealth.