It specifically says that the ex was on-board with the idea. And given that they were together for 20 years and early retirement is a goal that takes 10+ years

It specifically says that the ex was on-board with the idea. And given that they were together for 20 years and early retirement is a goal that takes 10+ years to reach, at a minimum, I'd say the OP did their due diligience. The read I got was that the reality of retirement didn't suit the ex's personality. The lead-up was fine, being frugal, aiming for a long term goal, but the reality of it had unintended effects that the ex couldn't handle as well as OP.