It depends a lot on the startup. I have similar number of startup experiences, and only one had early stage secondary sales ( but those were even for non founders ). Mainly money comes from IPO or other exit.

How has your time in startups panned out? Were those 2 successes worth the failures?

100%. The one thing you get at a bigco is bigness -- you can release a product to potentially millions of customers on day 1. When you call on a possible customer or partner, they return your call immediately because BigCo is on the line.

On the other hand, I can't stand how corporate politics corrupt a product's vision. Even low-level politics like "strategy taxes" are infuriating.

You don’t necessarily get offered it, you demand it as a term. If it’s a “hot startup” you pick and choose your investors so if some don’t like it they can walk.

Secondaries have become more popular over the past few (5?) years but you need to know to ask.