Earlier quoted context omitted.
OP is mistaking positive feelings (short term, extrinsic) with wellbeing (long term, intrinsic). Their excitement probably allows them to neglect any underlying problems. When the excitement wears off, issues that haven't been addressed are likely to reappear. All of this is purely speculation of course, based on my reading of the post.
Wouldn't "a rewarding career" qualify as long-term, intrinsic wellbeing? Cynics may believe no such thing exists, but I don't think it's a universal view. Of course, there's always risk that the job changes and becomes worse. In a Series D company, we know that could happen at any time. OP probably knows this too :)
Depends on what one understand by “rewarding”. If it's compensation, title, status, achievements, etc., these are extrinsic factors. Regardless, I don't see any evidence for a rewarding career in the OP.