I've been thinking a lot about this lately.
I have been Hodling for some years and I always thought that when we reached an ATH again I would sell some of it. So that would be around $20,000. I am supposing here that bitcoin won't have a major correction from now on until we pass that mark. What I think is that when we get closer to that mark many people will sell, but there will be many more people trying to buy, so I don't think there will be a major correction until the price clearly exceeds it.
In strong bull markets, price often overshoots major technical or psychological resistances like the ATH before supply puts a lid on price. Bitcoin, being a very illiquid market, is especially prone to that. If you'll recall in 2017, BTC reversed a few bucks above the 2013 ATH before a 37% decline in January. Then in March, it pumped 15% above the old ATH before another 34% decline back below it. Not saying it'll happen exactly that way the next time around, but it's important to be prepared for similar price action.
I don't recommend selling in that area unless you plan to buy back shortly after. Once price gets marked up above the 2017 ATH for good (not just a short-lived wick above like January or March 2017) then price is going to go parabolic. It'll be tough to get your coins back if you hesitate to buy back.
In the short term though, if I were to ballpark it, $18-22K is an attractive sell zone. Definitely a good area to take profit on longs if nothing else. Long vs. short ratio and funding rates as we approach the old ATH should also give us some indication when a top is coming. For now the technicals and sentiment still show plenty of room for upside.