Probably after you retire or if you don't have any dependents. The moment you have dependents (kids), parents you need to take care of and the subsequent load of mortgages, university fees, medical insurance and what not... optimizing TC suddenly looks like the best investment compared to any other education.

I'm speaking from experience. I got a PhD while working full-time. I enjoyed it and it gave me a lot of perspective. But - if I'd spent even 1/6th the time LC'ing I'd probably have a much higher TC at this point :). No regrets - but its an amusing thought that kinda lurks in the background.

Any advice on doing a PhD while working? I've been considering doing that in the back of my mind.

Here you go.. I put this blog together a long time back - https://gautamdivgi.wordpress.com/2017/03/08/my-part-time-ph...

Belief in value of education beyond TC is (1) inversely proportional to the number of mouths to feed and (2) directly proportional to the outstanding equity in trust accounts

That's a weird thing to say on a website that values curiosity above almost everything else.