I think it's good to think about a couple things: rooting hormone can be a dormancy breaker, which is a bad idea if you want to keep your tree dormant.
Having it indoors will also break the dormancy and if the root growth isn't happening well, then the tree will lose a bunch of foliage.
Most spruces have a pretty deep dormancy and can survive a month indoors, but they need to be put outside immediately after. Because they've been outside for so long when entering dormancy, they're unlikely to get out of it this fast. However, once the dormancy breaks, there is no turning back. Luckily they are pretty hardy and can resist frost very well even when not dormant.
So the best chance you have is to put it outdoors, and hope for the best. Since the damage is already happening, you can expect half of the tree to die. Maybe it will survive, maybe it will not.
The next question is: how much effort and energy do you want to spend? Spruce recover slowly, so one or maybe two or three growing seasons away from now - so maybe 2026 or 27 - you can think about doing something with it. Maybe your local garden store has a bunch on sale right now, in better health, and those will be able to be worked faster. I like saving trees! But it has to make sense, if the root system was already crappy to begin with, like you said, then maybe it's better to look for something else. Bonsai is a hobby for the long run, sure, but if you can play with a tree tomorrow as opposed to waiting 2 to 5 years.. I think it's worth thinking about that.