Senior chihuahua bad breath after cleaning, why licking everything?

Many dogs, especially small breed dogs, are prone to dental disease. Imagine what your teeth would be like if you never brushed them (or flossed). yearly dental cleanings are needed and beneficial to maintain dental health but they cannot be the sole dental care for an older, small breed dog. There are many things you can do at home to provide Nacho dental care and this will help with his bad breath.
The first thing to do is have Nacho examined to make sure his bad breath is really just bad breath. An infected tooth, kidney disease and diabetes can also cause bad breath. A physical exam and detailed medical history will help to veterinarian to determine the underlying cause.
Here are some helpful links about at home dental care:
https://www.petcoach.co/article/dental-care-what-to-include-in-a-complete-program/

When brushing the teeth, pay special attention to the small teeth (the incisors) because this area is very prone to disease. Do make sure however to brush all the teeth.
The licking could be due to a bad taste in his mouth but it can also be a sign of nervousness or a self soothing behavior.

I hope this helps and please feel free to post any additional questions.