Winter when it is dormant would be the usual time for any root pruning/ repotting work but ash is very resilient so
I suspect they would survive root reduction while actively growing as well.
Pruning can be done any time
Wiring and shaping can be done any time.
Previous advice has been to let it grow. While I agree that a good wide trunk is desirable I'm going to offer some qualifications - Taper will always be desirable, in fact, I believe that taper is even more important than just trunk diameter alone.
Leaving a single trunk tree to grow unpruned will increase the trunk diameter but will not give taper. When pruned you will have a thick stump with a single large cut - not very desirable and it will usually take many years to grow new leader, branches and for the initial scar to heal.

a far better option is to grow the tree on but with judicious pruning.
Pruning will usually produce taper in a developing trunk. It will also usually give changes in direction which will give the trunk much better character than the usual lazy S wired bends. I have also found that pruning usually results in multiple leaders, each much smaller than the main trunk below but contributing as much, maybe more growth to the tree than the single original trunk. When the excess leaders are pruned off the scars are much smaller so they are less noticeable and heal far quicker.
Even if occasional pruning slows the trunk thickness initially you end up far ahead in the end with a much better trunk and quicker final development of taper, leader and branches.
If this was my tree I'd lop it now, probably just above that small low branch, transfer it to a much larger container to allow plenty of root space to speed growth then water and feed well over summer. Next winter a severe root prune to set up better nebari (I'd lay odds that the roots are currently tangled, going in all directions and/or at different levels) then repeat the process for another year or 2 until the trunk is sufficiently developed.