Hi Charlie! I’m also a rank newbee at the accordion. I’ve played the piano (and other instruments) for decades but besides having learned a bit of music theory and music notation there is nothing in my experience that makes the accordion left hand easy. I consider learning the Stradella bass like learning a foreign language!
Already knowing some key and chord structure helps but you can prob skip the piano and learn what you need on the accordion alone. The piano experience has has helped me with the right hand keyboard, with rhythm, and reading written music. But you can learn all that on the accordion with a little help and a lot of practice.
For now I’m playing around a little with the keyboard but really concentrating on that foreign left hand. What helps me is a book recommendation from Mike at Liberty Bellows:
“The Mighty Accordion“ by David DiGiuseppe. This book is perfect, teaching left hand bass and chords only! It guides a beginner slowly through progressive exercises that really help. What started out as bewildering (made worse since you can’t even see the buttons or the fingers!) is slowly starting to make sense and gets a little easier every day. After a couple of weeks I can actually hit most of the buttons in the current exercises correctly! (I’m up to page 31 in the 112 page book)
And if you need it the book publisher provides online audio for all the exercises.
As someone mentioned, practice very slowly. If you stumble, slow down some more! If you get frustrated, take a break and hit it again later.
BTW, at first I propped a mirror on the music stand which helped me see the finger position and “tightness” needed to play the bass and major chords with the 4th and 3rd fingers and get the feel of moving them to the adjacent buttons in the beginning exercises. (I didn’t need the mirror after the first day.)
I practice the LH exercises over and over then sometimes think of a simple song to play on the piano side (or pick one from a beginner’s songbook), accompanied by what I’ve learned so far from the exercises. Then go back and repeat the exercises.
I see there is also a 2nd volume of the book with advanced techniques.
Also, a hint: unless already very familiar with it print out a diagram of the Circle of Fifths which relates directly to the LH button layout and the chord structure of many songs. These diagrams are easily found on the internet.
In addition, I printed out this diagram for the 120 button bass (a subset of which works for the smaller accordions as well). If I get confused this straightens me out with a glance:

This also helped me as I started practicing scales in the bass. (I think there’s a bit of this in the Mighty Accordion book 1 but it’s something I want to start practicing now)
And finally, the instructor recommendation is so important. With just a couple of sessions my accordion “mentor” has already helped tremendously with basics like posture, strap adjustment, hand position, bellows control and more. When I recover enough from a RH wrist injury I‘ll start with some real lessons.
Oh, one more universal bit of advice for learning ANY instrument - if at all possible practice every single day, even if it’s just for a short while. Getting into the habit of skipping 1 or 2 or 3 days is a proven way to progress very slowly, if at all. This advice comes from more than a half a century of my own experience with various instruments and from every good teacher I’ve known.
JKJ