
Logitech G
G600 MMO Gaming Mouse
Customizable MMO workhorse, durable, but discontinued and pricey.

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Ordered and tested it a few days ago. The sniper button is placed way too far and the upper "tilt" button is basically unusable due to the weight required to click it. Using claw grip I was lifting the mouse trying to click the tilt button from the underside.
I went on this journey last year and gave up. I didn't like the aerox 5 enough to keep it. Felt cheap, the third button was almost too easy to press, but I never got around to finding out if I could get used to it because the mouse felt so bad in hand compared to all the other quality mice I own and have tried.
As someone else said, the Aerox 5 by Steelseries is probably your best bet. I'm not fully sure what you mean by commissioning a company to make a single mouse for you or what you expect out of that. Besides the custom shell and PCBs you'll also need custom software to bind that key to a keyboard button since Windows only recognizes mouse1 to mouse5. IF they'd even do that for a one-of unit, you'd probably pay thousands of doubloons for that and it'll probably be 3d-printed and won't be near the build quality of an OP1 let alone the software being bug free on the first attempt. If you want it to be an actual injection molded shell you could actually get a new car for the amount of money the tooling would cost and factory slot that it will take up.
Steelseries aerox 5 is a little close imho
I feel that. The aerox 5 had ping wheel issues but I love love love steelseries. They've never let me down. Thats like one of their largest flaws. And that specific mice if what i need for fps so I was trying out other things. I just got my v4. I want to try the pro x2 superstrike when it comes out but thats not until jan-march of next year. Not to mention its a heavier (65g) mouse. Guess we will see if thats to benefit or not.
Yes, and it's one of the only remotely "modern" mice to have more than 2 side buttons if not counting Logitech's G502 series and similar mice. Glorious isn't worth being in any discussion since their shit isn't even worth getting for free. If wanting modern "lightweight" though, yea that don't exist LMAO.

Logitech G
G600 MMO Gaming Mouse
Customizable MMO workhorse, durable, but discontinued and pricey.

VXE
R1 Series
Budget king: lightweight, great value, long battery life.

Hitscan
Hyperlight
Ultra-light (39g) for small hands; sensor is outdated.

Razer
Viper V3 Pro
Adaptable shape, optical clicks, but scroll wheel malfunctions.

Logitech G
G502 HERO High Performance Gaming Mouse
Durable, heavy workhorse with free-spin scroll; prone to double-clicking.

Ranked #1
Hitscan - Hyperlight

Ranked #1
Teevolution - Terra

Ranked #1
Razer - Viper Ultimate

Ranked #1
ROCCAT (Turtle Beach) - Kone Pro

Ranked #1
Logitech G - G600 MMO Gaming Mouse

Ranked #1
Razer - Naga Trinity