can you Sacrifice a unit that has become mighty due to assault bonus Might after scoring when reacting to a battlefield conquer trigger

No — not if the Assault bonus was what pushed it to 5 Might.

Assault only applies "while I am an attacker," and the attacker designation is removed during cleanup before the conquer effect resolves, so Assault has already switched off by the time the conquer trigger is on the chain (FAQ #167, #3686). Sacrifice's Mighty requirement is a cost, checked at the moment you play it, so at that point the unit is back to its printed/base Might. If that's below 5, it isn't Mighty and you can't kill it to pay.

If the unit's own base Might is already 5+, it's still Mighty regardless of Assault and you can Sacrifice it fine.