Money solves the problem of having enough kicks at the can. Getting product market fit will eventually happen. You’ll just run out of money before then, in most cases.
Bottleneck is that the market moves on in the time you spend pivoting, and eventually your inside information about what's hot and what's worth building gets stale, along with your technical skills. I know a number of other entrepreneur/retirees in the same boat - 5+ years working on various ideas, often after having a previous exit - and it never seems to result in a big company.
My startup is growing 5% every week for last 6 months and it took me 3 years to get here. If my bank account was bigger I could have done it in an year. and because I am bootstrapped I am losing my amazing tech team to highly funded companies that can give them 3x salary and stock options
What about buy an existing startup vs bootstraping?
Lots of people don't realize learning to read the market is also a separate skill that can't be learned through working for a company. It takes time and a few failures to wise up.