Veteran here - out the other side now with a couple of lovely (most of the time!) young adults.
They were both an absolute nerve-shredding fucking nightmare at various points between when they were born and now (and I had 7 full years of chronic sleep deprivation with DD) but I have to say, on balance, I remember their childhoods with huge fondness and nostalgia rather than relief it’s all over!
When you’re in the trenches of a difficult period that’s testing your last nerve, all you can do is try to stay calm and consistent, and keep in mind the old saying ‘this too shall pass’. Parenting is like clinging to a lilo in a shipping lane - you just have to hold on and ride the waves as you lurch from one battering to the next, and appreciate the peaceful lulls in between!
One of the very last things my terminally ill grandmother said to me, when my children were still small, was to really be in the moment with them and try to appreciate every minute - hold them tight, breathe them in, listen to them - because you will never, ever have those days back and one day you might yearn for them.
I don’t think I really took that advice fully on board because, like you OP, at the time I was right in the thick of it. But I now totally understand what she meant, and my heart hurts a little bit that those days are gone. I’d give anything to have them both be 4 again, just for a little while.