I’m new to bonsai and have a Metasquoia - I love the curve of the main trunk but wonder if I might wire the central straight limb to give more ‘shape’? If yes, what kind (thickness) of wire and when to do this please?

To wire or not to wire is a question about different preferences. Some like to  clip and grow and some like to use wire. There are bonsai artists that has been in the hobby for 30+ years without using a single piece of wire.

Regarding your question about bonsai wire size, that is best determined trying to bend the branch with a straight piece of wire. If the wire bend before the branch it is too small. If the branch bends before the wire it is a good size.

In your partiklar case, I would concider the straight trunk a sacrificial branch and would just let it grow freely. No pruning nor shaping. But it is your tree.

I hope the tree was just indoors for photo purpouse? This is a strict outdoor tree and will not survive indoors.

Many thanks indeed for your reply and, yes, it lives outside.