There is no such thing as "soundproof booth" really: you can take a hefty amount of decibels out of the sound levels but if someone wants to feel offended, they still will. They are of course a viable solution where the noise level is a reasonable source for complaint and will be quite effective if there are regulations or a verdict prescribing you to stay below a level of noise measurable at the neighbors. However, when the complaints are more focused about the quality or unwelcomeness of some acoustic disturbance, you need to squeeze it below the level of detection, and that's not realistic.
The most effective solution would likely be MIDIfication of your acoustic accordion in parallel with getting some "null registers" (typically by sacrificng some existing register left as well as right and having the sliders modified, but conceivably you can also get an override added in a location not easy to access that puts the null register in comparable to how a Master switch or chin switches may work).
Except for the air use, that would give you a practice instrument that is very relevant for acoustic play. Also playing through an arranger then is a possibility, and that can be fun and help with training to keep accurate tempo. That, in turn, may mitigate your neighbor problem even when occasionally doing acoustic practice (prohibiting any musical practice tends to be frowned upon by courts): uneven and sloppy play tends to be perceived as more of an aggravation than something in straight speed.
It's not a cheap option, and it is less flexible than a separate electronic instrument. But assuming that you very much have a dedicated instrument you always play (which is not a given!), it may be the most straightforward way.
Is there maybe some cellar or garden shack or whatever in easily accessible distance that you could move to for practice? Not playing next to your neighbors is of course the simplest solution.
