I do find that I have to bend my thumb and forefinger quite sharply to get my fingernails into and pull down the slide lock tabs. I can see how that would be difficult for someone with arthritis. Unfortunately, I don't know of any extended slide locks. Also, if the tab protrudes beyond the frame, it could be moved when the gun is holstered.
I can think of a couple of possible solutions. There is a little bit of room -- not much -- to play with the width of the slide lock without its protruding beyond the frame recesses. A good gunsmith, machinist or other metal fabricator might be able to make one, perhaps with coarser serrations. Also, being only plastic, the recesses can be widened and flattened out a little bit -- again, not much. Or, devise a tool to reach up under the frame and pinch and grasp the serrations. A normal plier, even a needle-nose, is not going to work, as it will contact the bottom of the frame and not touch the slide lock. I have a plier used to remove the push pins that hold splash shields and other trim parts on a car that might work. Let me get it tomorrow and try it out on my own Glock, and I will get back to you.