I called my in-laws by their first names. My wife called my parents by their first names. My brother’s wife, who had a strained relationship with her own family, called my parents Mom and Dad.
I’ve never encountered anyone who called their in-laws “Mr. XXX” and “Mrs. XXX”
I have a similar age difference with my MIL, but she never stops insisting I call her Mommy. That’s crazy talk, so I don’t call her anything.
I told my own DIL to call me by my first name, but also said that I’m fine with whatever she wants to call me. So far, I notice she’s avoided calling me anything. It’s an age-old solution.
I’m pretty sure she’ll call me whatever grandma term she chooses once the grandchild arrives.
My FIL law was married 5 times. The family joke was that we were going to call his wives by number instead of having to learn so many names. When I’m being facetious I call my wife’s mom “MIL #1”.