What's the right time to transition from postdoc to PI?
If you do get a job offer, then yes, I think you’d be slightly crazy not to take it - more precisely, to not take it based on the concerns in your question would be a somewhat irrational decision. However, one thing you could do in that situation is negotiate a late start date that would give you time to continue your postdoc for X more years (assuming that is an option that’s available to you). I have seen this done and think this has a reasonable chance of success with X=1. For higher values of X I have never heard of anyone doing it, so I suspect the chances are not high, nor do I think it’s particularly advisable, but if it’s important to you you can still look into whether that’s a possibility.

In general, while you are somewhat correct in thinking that when you start a tenure-track position you become less “free” than a postdoc, and have more responsibilities and duties and therefore a bit less free time than a postdoc to explore and learn new areas, I think that’s beside the point. All the things you can do as a postdoc, you still can do, and in fact are still expected to do, as a PI. You also start acquiring important new skills and abilities that enable you to take your research and career to the next step. If you don’t feel ready for it, that’s probably because a lot of people don’t feel ready for it, so that’s completely normal. But regardless, the best time to do it is whenever people judge you are good enough to offer you a job that appeals to you. Good luck!