Miniature Schnauzer 13.5 years old, shaking constantly

by Angela
(Guatemala City, Guatemala)

I have a female miniature schanuzer, Luna, who`s been with us for 13.5 years. She has changed a lot in the last years, specially the last 6 months.
She became a lot more sedentary (she used to be super active all the time), sleeps a lot, she snores when she is asleep (never did it before),she shakes constantly, her hip and back paws are stiff.
I imagine some of these symptoms come with age...? Is there something you know about the shaking and what to do to make her more comfortable? I give her a tablet for her arthritis every day to help with the hip and the stiffness.
Also, a blood test showed the lieber enzymes are higher than normal, so we changed her food (she hates this new food).

Any advice on how to help her with these symptoms, the shaking, the arthritis? Any advice on what to do to balance her lieber enzymes?

I adore my Luna, she is a very special dog, and I would like to do everything I can to keep her as comfortable as I can.

Any advice will be greatly appreciated.
THANK YOU! looking forward hearing from you.