"I need to have as much control as possible over what I work on, if my work seems pointless it's a huge demotivator for me."

There are non-founder roles at companies where you can have complete technical control and a lot of product control. Growth hackers / technical marketers, data scientists and analytics people at small and mid sized companies, for example.

Coming on as a first engineer is another good option to tick this box.

"I want to do something meaningful"

Would working on a google x project count? What about an early engineer at someones funded and ambitious startup? It's not easy to get those roles, sure. It's also not easy to start a company working on something extremely meaningful. As a first time entrepreneur you're often going to need to work on an iterative solution in a more proven market because investors won't fund you on pre-traction moonshot ideas, and moonshot ideas don't often have early traction. For most people, unless you have an unusual background or set of connections that door won't open until you're a successful serial entrepreneur. What would happen if you took 1/10th of the energy and time you would need to start and succeed at your first business and instead use it to deliberately find job opportunities, learn relevant skills, promote your professional identity, and network in industries you care about? If you can get a new job or a significant promotion every 12 months, you should have no problem getting to a highly paid and meaningful role with autonomy. And all without putting your finances on the line.