NuVinci continuous shifter
The NuVinci system is quite interesting. There is no spring to pull against in the shifting mechanism, the shifter relies on two gear cables. You could reduce the required effort by using the best quality cables and housings available. There is resistance on the shifter though, not dissimilar to a normal grip shifter but without the clicks to power over. Whether it would suit your application very much depends on whether it's easier by a big enough margin to be noticeable.
NuVinci changed their marketing name to enviolo fairly recently. The products are basically the same though. https://www.enviolo.com/en/
Like you might expect, you'll only know for sure once you've tried it. Does anyone in your area use one of these systems?