How feasible would it be to hold my company over the barrel and try to get a mega salary increase? 2x type of thing?
It may be possible if you have a lot of knowledge about buggy spaghetti code and undocumented requirements that'd be hard for a brand new team to ever get their heads around. It's less likely if your team used standard technologies in a standard way and have clearly written well tested code. Either way, it's a bad idea - it could encourage your dismissal in some geographies. Even if you manage to get a huge rise now, you may well be dismissed as soon as a new team are recruited and have some basic training.
My other alternative is to basically quit and focus on appearing to be working while I leetcode.
You could do that. Or you could view this as a clean slate with the company, ask for more responsibility (with an appropriate pay increase) and use this influence to address whatever issues caused your colleagues to leave. If you want to change development processes, the technologies you work with or even the type of people you work with, you'll never have a better chance. This way you can help rebuild the team in a form you'll enjoy working in and get a pay increase.