if you're looking to fix mouse3, one frequent cause is gunk getting stuck in the spaces around the wheel inside the mouse.
Quick and easy fix is open the thing to clean it up with a toothpick and some cotton tips dipped in isopropyl alcohol. screws are usually behind the mouse feet, however you might have difficulty getting the feet back on properly, so you could be in for a new set of feet. Not a bad upgrade either, as after market feet are way nicer than stock feet on Logitechs.
If looking for a new mouse, tell me the size of your hand and the type of grip you use so I can recommend something really comfy. Trust me, shape makes all the difference in the world. Pics at the bottom to guide you. Judging by what you've previously owned, I'm pretty sure you're a palm grip type of guy.
I don't mind a bit of palm grip myself for comfort when just doing daily tasks or poker, tried all the models you mentioned and personally never felt comfortable with either. Deathadder seems a bit big and thick in the middle, even tho I've large 20x10 cm hands and MX518 shape is just poorly designed imo.
Most comfy palm mice I've tried followed the shape of Zowie EC series
https://zowie.benq.eu/en-uk/product/mouse.html
EC series will range from suitability with mid to large hands, if you happen to have small hands, you will need something smaller and there are plenty of options there too. Zowies themselves are good quality but have some problems with the wheels. If you're interested, I can recommend these or alternatives.
I also think it's worth exploring symmetrical shapes, granted, not quite as comfy in day to day use but amazing for fps shooters and such. Zowie FK shapes are fantastic in this regard, but most of the higher end gaming mice released lately are symmetrical so there's loads to choose from. To add, it's really helps RSI to switch between differently shaped mice once every couple of weeks or so.
One more thing, if you've always had heavy mice, you owe it to yourself to one day try light ones, it's so much nicer to move around 70g than 120g. If you manage to nail down a really good shape with a light weight, it's almost going to feel like you've never used a mouse before in your life. That MX518 is literally going to feel like a brick.
Hand/grip guides
https://dmarket.com/blog/top-gaming-...75_lanczos.jpg
https://www.trackballmouse.org/wp-co...-measuring.png