I've watched Drop Loyalty and Pride and I think it's a fine video for people who want to amass a pile of Ws, but ultimately it's just Chase the Meta: Splatoon Edition.
I should have stated that this wasn't about winning matches. I'm not looking for "pick a different gun" advice. My goal is to overcome this skill plateau, not ditch it for a different one.
First of all, that's a very cynical and downright rude way of looking at this video. Dude is right on what he's saying and he means well with that. The meta is there for a reason and it certainly doesn't hurt to look at mastering one or two weapons which are in the meta and will most likely stay there if you want to get into competitive Splatoon - but that's beyond your topic, so I will move on.
Now if you really want to hear something about getting better with chargers, buckle up and lean back and be confused:
... master a few more weapons. Or at least get them to a skill level where you can
honestly use them in a league/ranked session.
"Wait whut, Libra are you crazy? Aren't you like a full-fledged charger main since S1?"
Yeah, but hang on there, I have a reason for saying this.
If you learn more weapon classes, these skills and movement patterns will transfer to your Splatterscope play. Like, play another long ranged weapon which is more flexible while still restricting in close-range combat (Jet, RPro, to name two) and learn range management with those. Play other weapons of shorter range to get a better feel for their effective range. Why is this important? So you will know, as a Charger, who is pretty pooped if staked against a high-level player in close-combat,
when it's better to disengage and reposition before sniping them off. This is much easier done when you have a much better idea of what the effective range of these weapons is. Certainly can you learn that from fighting them too, but when have a larger pool of weapons at your disposal, you pick up even more. Like, say, their movement patterns and approaching routes. It sounds super counter-intuitive to go all out of your way just to overcome a skill plateau but hey, it worked for me to overcome my rust, so might as well share.
Other than that, anything else that I could think of has already been said. Practicing "quick" snap shots and "trick shots" is a must on certain maps to keep the upper hand since a few stages are really the horror for Charger mains (*coughs*Starfish*coughs*) but if you're really sure about this, you can get there. Best of luck to you!