Well, it's a wish list but thinking it through, I don't see how a master button would work other than on a melody. However, if each button ( landing pad ) had multiple functions, like a little joystick. an 88 keyboard could be reduced to 52........😊
Playing for that long at a stretch, especially as a beginner, is counterproductive at least for me. Unless you're a cyborg your brain will be overwhelmed and stop processing after about 20 min. or so. Roughly 20 min. every other day is optimum for my particular lump of grey matter. With your sore arm I'd stop as soon as it starts to hurt. Combine that with Alan's suggestion to play quietly to give both your brain and body a chance to recover.
For a visual, search Wakker concertinas bellows. Concertina connection also has some info. There was a discussion here a couple years ago but I can't find it at the moment.
I have one duet with Anglo style bellows and one with EC style. The first give a bounce back or nudge upon bellows reversal which I like because it lends momentum to dance tunes and helps with quicker ornamentation. With the other it seems like I must come to a full stop to reverse but that is useful also. I like 'em both. They're just different. Thinking about it.... I play the first in a more closed position and use the push back to start triplets and such and the pull to start new phrases or modulations. With the other I play more stretched out with altogether less bellows changes. sorry I'm posting over you Robin.