First of all, I dont think that Slavs are weak or op. Their balance is quite ok.
What disturbs with them is the way how they balanced them.
After all, cost reduction on Druzhina and Boyar barely makes a difference, sonce both things come very late or not at all in most games.
Detinets is far better than orthodoxy but effects the second castle at best, so the impact is also not the big deal.
What made the point and gave Slavs their recent push was the restoration of the old farm bonus.
Anyway, balancing with a passive farm bonus is still a bit dumb and nothing really changed with the lack of infantry identity.
Considering this, I would do some Changes to Slavs:
a) supplies and gambesons not only free but available one age earlier. This would make an interesting MAA rush and after first armor upgrade, Slavs MAA would be simiar to Romes. Together with Druzhina gives some good infantry identity.
b) Give them a bonus to get first castle cheaper and remove chevalier from their tech tree: This would be interesting for arena and some rush or defence strategies. In return, knights wont work in the long run and boyar gets a viable replacement that is available nearly as early as knight. Having 2 units that similar is boring anyway. Besides that, there could be more civs in general, that share the unupgradeable knightline with saracens.
c) redesign their archery range. For what does a civ without barcer, thumb ring and parthian tactics need heavy ca??? Civs with bad archers usually get hand connoneer. With that, the ring archer armor would finally make sense and Slavs would have a source of pierce damage outside siege. So remove heavy ca and replace it with hand cannoneer. Still one of the worst archery ranges but at least sometjing to do with the building, after early game skirms.
d) since the first 3 points would be straight buffs, just lower the farm bonus to balance it out. The changes should not result in a powerhouse civ, but one witv some a unique options instead of strenght purely through a boring passive eco bonus.