Sounds like you're considering leaving your current role... have you considered telling your boss exactly what you've laid out in this post and asking for more challenging, engaging, and rewarding assignments?
(What's [s]he going to do -- fire you?)
I usually love boring code bases, because it generally means it's been used for a long time and the stress level is usually pretty low. Are you only burning out because it's boring? Or other stresses with the job?
I can relate to this.
I think this is a question on what you want to pursue. I want to say choose carefully, but to be honest, it doesn't matter that much.
the important thing is just that you pursue something.
if technical challenge is what you want to pursue, consider planning for and fighting to move into compiler teams or kernel teams (maybe find a new job that can support that).