You shouldn't have any business with when the 21 million supply of bitcoin will be reached, what assurance do you have that you will still be alive by that time? Another factor is Bitcoin halving, slowing down the numbers of Bitcoin that's getting out into the hands of others through mining, Erase this plan or idea off your mind OP and focus more on your dollar cost averaging for 6-10years, this must not stop you from living your life, because we don't know when our time will be up.
As Bitcoin supply gets very close to a 21million, things will get extremely slow, if I have to start DCA into Bitcoin right now I will target 6 - 10 years later, I have to take profit and do some buyback, I don't like holding through a bull market and not taking profits, a bear market will always bring down the value of Bitcoin to a point for future buyers.
Don't worry about it now, just stick to your strategy and work towards your goals. It is possible to take profits in a bull market and buy in a bear market, but I am sure that it will be difficult for you to determine when to sell and when to buy, but the profit may not be great if you determine these levels incorrectly. if you decide that 100K is a new ATH and sell it and the price reaches 300K, it can deprive you of a big profit.
