I've got 2 yoe in high performance c++, working on data processing engines. I'm really tempted to go to the startup... But big tech pays more. And I feel like I could actually learn how to use tools properly before I come back to writing them. Any opinions?
Big tech isn't going anywhere. If you are getting offers from big tech now, you will later as well. I always tell people to start at smaller companies. They are way better when you are young because you will face a wider array of problems, which builds better experience. Big Tech is great when you are older. It is generally more focused, higher paying, better benefits, but can often be less rewarding.
But my point is that, you can almost always find some way into an interesting company such that you may be able to eventually pivot to (or at least periodically work with) a team you're interested in.
The difference is probably that it is harder to get those jobs because everyone wants to work for the flashy exciting companies. I was considered for a job at Docker a long time ago (when I was an early-midlevel) and the recruiter told me that over 1,200 people applied in 5 days. For entry-level/intern positions I assume it is similar or even worse.