Very much just Oom-Pa or Tango / Swing like style (chord + bass at the same time) plus a few simple runs.
I feel that you are searching for something that doesn't work on the accordion. As a piano player, you probably would like to play walking basslines or bass grooves.
The notes of a piano have their own natural decay and the percussive nature of the hammer helps to feel the rhythmic pulsation of the piece being played. On the piano, you can play the bass part and add some backing chords with the left hand while the right hand is free to play the melody or improvisation. The pedal offers the possibility to extend the decay of the notes and allows to fill the space between one note and another, the harmonic resonance of the piano strings also helps to fill the gaps.
Now, try to imagine playing a Hammond organ, a Wurlitzer, or a simple electronic keyboard with a dry, non-decaying sound. You can't play the same way as you would on a piano. You should adapt to the characteristics of the instrument.
On the accordion, you must try to obtain both rhythmic and harmonic accompaniment knowing that the sound depends solely on the air pumped by the bellows and on the articulation of your fingers.
So imagine having to play the bass part, the guitar part, and the lead part with an instrument that stops making a sound as soon as you take your finger off the key or as soon as you stop pumping air into the bellows.
The only possible solution is a rhythmic-harmonic pattern interspersed with short simple melodic runs of the bassline.
That said, there may be exceptions: it is possible to play walking bass with an accordion
but it is a stylistic choice that is not always feasible and in any case more complicated than other more rhythmic solutions (and in my opinion, more pleasant to listen to):
Obviously, in the case of a midi accordion, it is a different situation
