What will you do when Bitcoin reach$20k?
If we're going to be honest... then probably nothing, because I'm not confident enough in timing the market to either buy or sell at that particular price.
But if I was the kind of person to bet on this stuff, I think I'd consider buying a straddle option.
Given the history, I somehow doubt the price would stay at $20k for long - judging by past behavior of this market, I'd be more likely to expect that either there'd be a new ATH and bull would officially be back in season, or it'd "fail to break through" and a bunch of old money would push the price down by realizing their gains.
If I could find one at a reasonable price, a straddle seems like the right tool to make a bet on that outcome.
Do you have a price in mind than when bitcoin will reach it, only then you plan to sell?
This is a more interesting question.
I think it's a good idea; having a plan makes it easier to keep a level head in a volatile market like this.
But I think having a price in mind is not enough - deciding how
much you will sell at that price is just as important.
I don't think it makes sense to have a price to sell 100% of your BTC at, if you are not here purely for trading. I used to post "current money or bust" a lot on these forums. These days, I'm less confident that BTC is shaped properly to become "current money" (i.e., something that circulates like cash and can be used as such), but the point still holds that nobody really knows what the future of this thing will be, so holding back a little of your stash "just in case" seems smart.
So barring emergencies, a better strategy is to say "at these prices, I'll sell these %s of my BTC", picking numbers such that it'll be "worth it" in your real life. For example, an amount that will let you
FIRE without
relying on future BTC gains. Or to put a down payment on a house, or so forth. Something concrete that the money will do to improve your life, above and beyond "well I guess I have a lot of USD now".
But that also means the amount isn't necessarily static! You'll have to adjust them if your IRL needs change, or the amounts necessary increase/decrease. And those adjustments must be kept independent of whatever the BTC market is doing; the point of a financial plan is to keep you from getting swept up in the market's emotions and doing something irrational.
That's my take, anyway.