Thank you for your detailed response. I am 37 now and plan to ‘get up to speed’ through private study then start a PhD when I am 40. Did you remain in academia after your PhD?
Think about how private study is going to translate to your application. Talk to professors to see what you'll need to get admitted. I had roughly the same plan, but luckily had dropped out of my undergrad so I was able to go back and do a 2yr undergrad right before applying to grad school.