No one at work is going to let me learn Lisp :)

But if the purpose of doing side projects is to be more competitive in the job market, are there that many high paying jobs that want Lisp?

Besides, in my experience, knowing a specific technology from doing side projects usually doesn’t count as much to potential employees as having on the job experience unless your open source project is really popular.

I would view any Lisp project as a major bonus in favor of a candidate. It's really encouraging when you see people who have an interest in different programming languages. When I look at people's open source, being a polyglot is a huge plus. Each language offers a unique perspective on programming and the desire to explore them is a strong signal of a self-learner and someone who is passionate about tech.