Your description seems to indicate that you are keeping the left end of the instrument pretty much fixated to its position at the left leg while working the bellows dominantly with the right hand and doing so you get too much static load
engaging the left thumb since you also probably are stabilizing the instrument almost entirely with your grip.
If this assumption is not entirely correct, let's make a re-start....:-)
The onesided static load is one problem and using the thumb for it another.
Generally speaking common 'work hygiene' like frequent breaks is essential but since the problems related to static load may appear already within one or two minutes you can not safely avoid that in common music practise....playing tunes shorter than half a minute at a time maybe....:-)
So in principle...
1) find ways to work symmetrically or by alternating bellows work regularly, frequently and rhytmically between the two hands.
2) find ways to avoid gripping the instrument at all...let the hands/arms do the job for management of the bellows instead of the thumbs (or any finger in case of English but this is Anglo I guess..or Duet)
3) find ways to avoid carrying any weight of the instrument by muscular force or strain of joints...your own left side as you describe it ought to be safe in this respect but the right hand does 'carry some weight' ...although with the the Anglo there should be small problems with this except when standing
On the other hand it is easier to work symmetrically while standing which is an advantage.... BUT
4) the original handle (wooden bar + handstrap) is very primitive and could be much improved to offer better handposition, better stability of the connection between hand and instrument, and better relaxation for fingers....you may find
this aspect developed in some articles:
http://www.concertin...an_ergonom.htmlhttp://www.harbour.d...q/rahm/rahm.htmhttp://www.concertin...an_holding.htmlhttp://www.concertin...supporting.htmlhttp://www.concertin..._supporting.gifGoran Rahm