>"the 5th gen F-22 level engine isn't happening in the near future and thus they need a plan B"
Accordingly to online sources they are actually testing second stage 5+ gen engine for SU-57 and are hoping to deploy it sometime in 2022. Will they succeed is a different question of course.
"The Russian military will be supplied with 76 jets by 2028, 22 of which will be operational by 2024."
Even if they deliver on those estimates, i think the skies will be different in 2030-ties (imagine the thousands of Starlink like satellites managing the army of drones - each drone is easy to shoot down, yet no human carrying plane can defend itself from a large group of drones simultaneously firing beyond-visual-range missiles) Anyway, i don't believe those estimates for a lot of reasons. In particular most of the Russian new military hardware - i mean generationally new, not just modernizations - that has been shown and planned for deliveries starting in the last decade hasn't yet materialized, with the money being one of the key reasons. Another is that Russia is still not going to be able to project power anywhere beside its own vicinity, and thus will be focusing on protecting its shores which, given limited financial resources, favors as the first priority ground based advanced anti-aircraft defense and MiG-31 style approach (ie. like SU-35 with advanced radar and missiles instead of going all the way with Su-57).
Accordingly to online sources they are actually testing second stage 5+ gen engine for SU-57 and are hoping to deploy it sometime in 2022. Will they succeed is a different question of course.