Working on your language project is probably not the best avenue to get you out of your job, since monetizing something like that is pretty difficult. It's been done but it had better be a 'game changer', another variation on some theme is not going to help you become independent.

The abstract knowledge you pick up (such as algorithms, you already refer to that) will help though, since it will allow you to build relatively complex stuff where 'cut-and-paste' types will have a disadvantage competing against you.

Open source is great way to advertise your skills, but only if you want to be re-hired somewhere, is that your intention or do you plan to become independent ?

The last item on your list sounds like the best bet in the shorter term. But maybe first you should exchange your job that you hate for a nicer one to get you out of the frame of mind you're in and in to a more healthy one.

Then from a position of strength you can make your decisions, usually that works better (at least, it does for me).

Good luck!