Going into dormancy without having built any vigor during the grow season is definitely a concern, If the greenhouse is something that you keep sufficiently warm all winter it could be worth the gamble to deny it a dormancy in the hopes of it convalescing. It does seem odd that it hasn't bounced back following a re-pot though, have you checked if the drainage holes are clear or if there are air pockets in the medium?
I think theres a few factors playing into this tree never really taking off and being able to push new growth.
The repot I did was a little later than I should have done, I did very little root work and figured it was in need of it so it would be overall better for the tree.
My first season last winter with a “greenhouse”, a hand built structure wrapped in 6mil construction plastic, yielded good results at first, with plants coming back quicker and with more vigor than before, however, it got hot pretty quick in there, which fried out a handful of beautiful larch Forrest’s I had. So the “great fry-ening of 2022” probably didn’t help this maple.
The leaves it had in spring, which was a full compliment, slowly started to wilt and die off, with very little leaf from mid July till now. The water mobility was very slow, but I was still trying to figure out if the leaves had died because of the heat or water mobility.
I think I’m going to take my chances with it in outside greenhouse , I will lay down some heat mats and place this on top of one to prevent the roots from freezing