Thumb straps according the the traditional concept usually come in two variants:
1)comfortable but offering very poor stability and control
2)uncomfortable but offering a trifle better stability and control

In a way the problem is unsolvable since this traditional handle concept - thumbstrap combined with finger rest - is a contradiction in itself.

The thumbstrap idea (compared to the handstrap idea of Anglos and Duets) offers excellent access to the whole keyboard and this of course favours playing a plenty of notes. The tonal control of these notes however is suffering due to reduced control of bellows work - i.e. the capacity to articulate and playing with dynamics and expression is poorer due to the floppy handle.

IF(!?) using a thumbstrap as the major "handle" at all I mean that it is possible improving the situation at least a little bit by:

a) making the thumbstrap broader and steadier than usual
b) making it more comfortable and thus admitting a tighter fitting

add a) I use 25-30mm wide straps (compared to traditional 18-20mm) which means that the distal thumb joint is immobilized and I use a steady cardboard 'sceleton' making the strap fairly stiff

add b) by polstering the inner side richly (thickness 5-6mm) it fits tight but still comfortably and the instrument may hang passively on the thumb without any assistance by the 4th finger

Something to discuss with producers of "traditional thumbstraps" ?

Göran Rahm