Help needed: Lemon Tree

Hello everyone! My wife has what was a fairly well established lemon tree. After the birth of our second child it got somewhat neglected, the greenhouse got badly damaged, and I think the frost got to it. Either way it looked visibly dead except that when the trunk was scratched, there was green underneath.

We moved it inside over the winter and it sprouted a new growth, which has, as you can see, reached an impressive height and is still going.

What I’m seeking help on is what to do next? Should we remove the old trunk? What do we do to encourage it to bush out? Many thanks in advance!