Who has the best/longest warranties on carbon frames these days?
What are peoples experiences like of warranty processes with companies after the bike is a few years old?

Surely you want to be asking which manufacturers people haven’t the experience of claiming on warranties?

“If you’re not getting the right answer maybe it’s because you’re not asking the right question”. I can live with that, lets ask a different question….

Which manufacturers high end carbon frames have the best reputation for not breaking, and when they do break, which manufacturer has the most reliable warranty with the least small print and excuses for rejecting the claim?

Experience of claiming? None
Though picked Santa Cruz partly due to a 5 year warranty on the frames.

I’d not touch anything less than 5 years, also be aware that most lifetime warranties are actually 5 years.