If you don’t build your dream someone will hire you to help build theirs.
I always had a feeling that being an employee is just only about selling your time. It's no all about how interesting your job is or not, it's about the value you're building is not yours.
Setting up a scene
Imagine that you're working on a very small startup, which has a revenue of $50,000 per/month and your salary is $5,000 per/month and there are two other employees together with you and they're getting the same salary what you get.
Your product owner/company owner/CEO outperformed and also the development team did a very good job so you did to increase your startup revenue and now after 1 year it's revenue is $500,000 per/month, also the team hasn't been increase as this product just doesn't need huge team.
No matter how good that product can perform, your salary will stay same (maybe you'll get some bonuses, little-bit increase, but comparing to product revenue that's nothing) In this example your salary isn't binded to your company profit when it's growing, but it is if the profit is lowering (Judge by yourself, is it fair? 🤔, but that's how it works. You would do same, that's how employment system works).
My point is that you build some product for a company, try to maximize the value, but the outcome is just a salary you're getting monthly (If you're lucky you'll get a bonus).
Working on a side-projects are mandatory. Even if you have only 3-4 hours per-week to work on them.
With you own project, you build a value, which is yours. Over-time it's value will increase and it means: your value and income (of course if the product is profitable) will increase as well.
TLDR; work at someone to have basic income. Work for yourself if you would like to generate a value for yourself.