How do you integrate and arrive at closure in a healthy way when you've finally achieved something profound that took a huge toll on you and almost destroyed you many times along the journey?

Time, and patience. There's no rushing it, so might as well enjoy living slow and thinking slow. No need to finish every thought

First, I take a breath. More often than not, I will take more than one.

No matter what I'm doing I will always make some room to breathe. At the very least, I will go out for a long walk every single day of the year. Just walk. No distractions, no social noise, no music, no podcast, no nothing but me, my head, and that tiny thing called 'the world outside' ;)

Even better, when whatever it is I'm doing is stressing me out, I will make it multiple times a day because I don't work well when I'm stressed, so it's not a waste of time. And it also helps me stay in shape so it's even less a waste in that regard.

Last but not least, I learned to work less. It's not how hard you work or how tired you end up being that should matter, it's what you have done and much you like it ;)