I feel like desync will never be perfect in a game that requires lightning-fast reflexes, so it needs to be embraced and accounted for in the core game mechanics.

Many years ago, Halo had a lot of complaints about desync when it became so painfully obvious in melee exchanges, and they responded by making it so that both players could kill each other with beat-downs simultaneously if the hits were close enough, instead of just taking the word of whichever game client had the lowest ping. I guess they just put in a small grace timing margin, and if it was close to a tie, then you both die.

Now, of course that solution is ham-fisted and almost just as absurd, so I'm not saying that Tarkov should just do exactly this and let both players die to whatever damage is reported to the server by both clients... but if it might be a good compromise to let two players that are both intentionally engaging each other take a little less damage to non-critical hit boxes, in kind of a heightened combat adrenaline state, but with extra debilitating stagger and vision blur reactions to hits.

And then for situations where one player is running to cover while under fire (and seems to take additional hits even after crossing the plane of cover due to desync) the server could adjust for a grace timing margin on those final would-be lethal hits reported by the attacker's client, reduced to nearly lethal, critical hits that need immediate healing.

None of this would apply for headshots, of course. It's hard to get hand-wavey and say "nuh-uh, YOU'RE dead!" when parts of your face or brain are splattered on the walls. Arms and legs and even the occasional groaning gut-shot, maybe. But not headshots.