Here's another idea -- these thumb-only trigger shifters, again from Microshift. If finger-pull is the issue, these might be the ticket. If thumb actuation is the issue, couldn't these be mounted rotated forward as a finger-pull-only alternative? It may take some getting used to, but could be worth a tinkerer's try. Just a thought.

https://www.microshift.com/en/product/ts38-7/