Sorry for the late response.
A Doberman mix that is almost 12 is pretty old. As dogs and humans age there lungs age as well and become fibrous, meaning it takes a little more effort to breathe. You are doing a great job with her, there is not much you can do since this is a normal aging process. The snoring as well.
Now there lungs can change quickly so in 4-6 weeks from the last xray you should probably go in for another.
There are meds that can help open the bronchi, but since she is not strugglinig, I don't think I would start them at this time.
Good Luck,
Dr.V
Dr. D.A. Vamvakias
Veterinarian