Don't prune it. Let it grow to its natural shape which is particular to this tree and, as Dove says, you'l provoke it into suckering and will wish you'd never planted it.
Leave it alone, talk to it nicely and, maybe, give it a feed to compensate for all the previous hacking.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
Posts
In the sticks near Peterborough
https://forum.gardenersworld.com/discussion/1025263/rhus-typhina#latest
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Leave it alone, talk to it nicely and, maybe, give it a feed to compensate for all the previous hacking.