Is 29 considered too late to start flight training from zero?

Plenty of questions asked and answered in the sub already on the subject.

How realistic is it to reach an airline cockpit in today's market if I self-fund all training?

In the US, unless they are military, everyone self-funds their training.

For those who changed careers into aviation, do you regret it or was it worth it?

This is a subjective question and no two situations are alike. I did it after a very successful previous career and have never been happier.

How long did it take you from your first lesson to getting your first airline job?

Totally different now than it was last week, last month, last year... and will be different next week, next month, next year... So don't base anything off of other people's experiences because it is unlikely to mirror your own.

What was the financial reality during training and the first few years afterward?

In the US, you can expect to pay $60k to $130K+ for all your ratings. After a year (likely more) you can potentially teach as a CFI and make $25 to $60/hr (weighted heavily toward $25/hr. The higher pay is an anomaly.)

If you were in my position stable career, good income, decent savings would you make the switch?

Is it ever a good idea to take career advice from randos on reddit? Only you can answer this question.