A few years back I started having really bad arthritis in my hands on account of being an ancient fossil, so I had to go through a bunch of different gaming mice to find something that worked. I think I went through like 20 in the last few years alone.
To make the long story short, the Mionix Naos Pro is the single most comfortable mouse I've ever used, and it ends up being my every day mouse. But for FPS specifically I use a Logitech G Pro Superlight. It is a terrible name for what is a very good mouse, but it's about the right size, the shape's... fine, and the performance is nearly flawless. The side buttons are mushy and terrible and sometimes the wheel registers when I haven't moved it, but minor glitches aside it's been good, and I don't feel any wireless latency whatsoever.
Also I have to give a special shout out to the Xtrfy MZ1, a very weird mouse with a very weird shape, but quite nice once you figure out how to hold it. I've seen the grip Zy (its creator) uses with it and frankly how his hands haven't turned into arthritic pustules by now holding the mouse like that I don't know. Still, you can probably find a grip that works, and the very small size and narrow grip width has its benefits - I felt like I aimed very well with it, at least by my standards. In the end though, I found the G Pro more comfortable.
But do check out the Naos, the shape is amazing. It's a bit heavy at 100g or so and the shape is not the best for picking up your mouse a lot so I wouldn't say it's a terrific FPS mouse (unless you prefer a heavier mouse and don't pick up a lot) but it's great at everything else. Not for small hands though.
Also the Cooler Master MM720 seems like it should work as it's designed around the same concept as the Naos... but it doesn't. It's not even close to as comfortable, and I don't know why.
Other mice that stood out in my testing were the Mountain Makalu 67, quite nice if you want a larger ergonomic mouse, and the Roccat Burst Pro, if you want something medium and ambidextrous.