I can't point to any single financial decision. In my experience, it's more about the cumulative effect of many smaller decisions.
On further reflection if I can point to any one thing it would be keeping money and finances in its proper place. That is, a good tool to provide for needs and even some pleasures, but not what defines my life or purpose, and not something to be used for status. And I don't view "my" money as my own, but rather as a gift to be stewarded.
Likes others here, I saved a bundle in college by living with my parents while attending a community college followed by a local state university. I intentionally avoided the fancy universities because I wanted an education, not an impressive degree (not accusing those who went to Fancypants U of anything, this was just something that was important to me at the time). Graduated debt free, which meant I could start saving and investing in my early 20s.
Married a woman who shares my values, so we've always been on the same page with finances. Like me, she's also somewhat frugal but not afraid to enjoy what we've been given.
Have never really felt the need to impress with fancy cars or expensive houses. Even when I was making high six figures for a few years, we keep the modest starter home and old cars.
This has also made it very easy to give money away to non-profits (our church, but also homeless shelters and overseas aid groups). Interestingly, this has also made investing much less stressful. Giving away a significant amount of money annually puts the ups and downs of investing into a different perspective.
Because we weren't trying to impress anyone with our spending, and we kept lifestyle inflation in check, we were in a place financially where I was able to confidently jump from a stable career job at an established company to a (at the time) relatively unproven startup. This became a very lucrative career move.
While at said company, I was able to take risks that paid off handsomely. For example, respectfully informing a Senior VP why he was getting negative reviews from his reporting chain. This could have gone badly if the feedback was not well received, but instead he was grateful for the honest feedback and ended up rewarding me with a large equity refresh.
Finally, I was able to retire early despite some rather generous golden handcuffs, which meant leaving a very significant chunk of equity on the table when i resigned. I'm very very thankful I didn't spend the past 7 years of my life sitting in traffic and pointless meetings. Instead, I got to spend a lot of time with my family and be there with my kids as they grew from toddlers to now preteens. Retiring, instead of working just to make money I don't need, is probably the best decision I've ever made, but this was contingent on a bunch of preceding decisions.