There have been some articles in the Concertina Journal on topics related to routines holding the English concertina:

First a survey by Allan Atlas concerning the role of the fourth finger as described in some historic tutors:

https://www.concertinajournal.org/articles/the-english-concertina-and-finger-4/

Then some views from an ergonomic outlook were added by myself:

https://www.concertinajournal.org/articles/the-english-concertina-and-the-dilemma-of-the-fourth-finger/

Recently Henrik Müller described his personal modifications of an English by using just a handstrap similar to the routine with Anglos based on his interest in playing trad Irish tunes with his English in an Anglo-like manner:

https://www.concertinajournal.org/articles/no-thumb-straps-no-finger-rests-but-it-is-an-english-a-personal-journey/

Those articles all touch the general problems related to inevitable conflicts between needs for *stability* ( to pump efficiently and to articulate tone ) and *flexibility* ( to reach as many buttons on the keyboard as possible ).
All concertinas suffer from such dilemmas due to the habit carrying the instruments between the hands/arms while simultaneously trying to manipulate them in various ways making music....

Read, consider, and do discuss if you like..
Best wishes
Goran Rahm