Can a hero with assault that bring sit up to 5 might draw from sunken temple

Yes — as long as it's conquering out of an actual Combat Showdown, the Assault bonus is still up when the Conquer happens.

Assault is "while I am an attacker, I have +X [M]" (Rule 807.1.c), and the Attacker designation isn't removed until "Combat ends" (Rule 466.7.a) — which comes after Establish Control/Conquer (Rule 466.5.d). So a unit that's 3 Might base with [Assault 2] is a 5-Might unit at the exact moment it conquers, counts as Mighty (Rule 708), and Sunken Temple's "When you conquer here with one or more [Mighty] units, you may pay [1] to draw 1" triggers. This matches the Unleashed-era clarification in FAQ #12324.

The catch: if you moved into an empty/uncontested battlefield (a Non-Combat Showdown), your unit never gains the Attacker designation, Assault does nothing, and it won't be Mighty — no draw. Same if the +Might comes from anything applied after the conquer (e.g. a "when I conquer" buff), since Sunken Temple checks Mighty at the conquer moment.