Since your goal is to hold thinking on long term results (18 years from now), you should think about storing your coins safely on your own hands. That is, to not rely on any third party online platforms to keep your coins stored. You need a hardware wallet and to control your own keys.
Without doing this, I guess it's suicide to hold for so long while trusting all your money to someone else, as we saw previously during the whole year of 2022 with so many big scams happening around and investors losing all their money, including retirements from decades.
Apart from that, OP is also obliged to consider having heirs for his assets or bitcoins because no one ever knows how long they will live.
He says the time frame is 18 years, so of course this suggestion would be good suggestion for him to think about even though he knows back up wallet is mandatory. It's not really good to have everything stored on centralized exchange anyway, unless he trades daily. However, because he is planning a long-term investment, of course having a safe wallet is the solution.