Big Schefflera project

Hey, so i got this massvie schefflera. IT grew as a housplant, sadly the previous owners couldt Tell me how old it ist, so what would you guess (i put a pen in for scale). So its sitting under a grow light right now, waiting for spring to come and get hard pruned. So i am Not shure how deep i want to take the main Trunk down. Where would you cut it? I dont really Like that the trunk is so straight, at the Same time i dont want to reduce it so far down, as i am planing in keeping the tree rather large. And does anyone have eny experience with the healing Progress of big cuts on schefflera?