I just fixed the Joy-Con wobble that I've been experiencing on both sides. Just take the Joy-Cons off and apply 1-3 small squares of electric tape (less than 1x1cm) to the bottom of the rails of your tablet, and damn this thing feels rock solid now! I've had some annoying wobbling when in handheld mode, that I first thought was normal, but seems like this is an easy fix that makes the experience much better.
This is the second fix I applied to my Switch, the other one being scratch prevention on my dock by taping the insides with electric tape. I don't even mind having to do all this DIY stuff, it's kinda satisfying, haha.
Reminds me of my n3DS I got a few months, had this annoying plastic creaking in the back, so I had to remove the backplate and tape around the battery.
tl;dr: electric tape is the GOAT