I have a small, old Hohner accordion designed for a child. I have put a small dot of glue (I forget whether it was superglue or glass & bead glue) on every other bass button so that G feels different from D, etc.

It really helps. C, of course, has its own special feel because it's indented. I think of C as being like D and Bb; it's a button with a special feel

The glue is on only one row of buttons, the row that plays the tonic bass note. Starting at the bottom:

Eb smooth
Bb has tiny dot of glue
F smooth
C indented
G smooth
D has tiny dot of glue
A smooth
E tiny dot of glue

I was getting pretty good at accordion, and then I got spurs on my left arm. Bummer!