Exchange is the first place a new investor needs to store his bitcoin in the beginning because it is from exchange that he will buy his bitcoin from and because he is new in bitcoin and have the money to start buying immediately, he will leave his coins in an exchange and gradually when his bitcoin size is increasing due to regular DCA purchases, he can learn on which wallet that he can use to keep his bitcoin safe. You should also know that since he is a new investor and getting started immediately is the best no matter how little the amount is, he needs to pile up his bitcoin in an exchange so that it can get up to $500 and above before he can send it to his self custody wallet that is best for him.
If not for high transactions fees I see no reason why you should buy and accumulate Bitcoin in an exchange because it can become vulnerable to hacks, we should know that exchanges is not the best place to store our Bitcoin no matter how little it is because the security of our Bitcoin should be taken at utmost priority. I know that $500 may not appear too big for you but to some people it is especially when they think of the amount in Fiat they used to buy such quantity of Bitcoin so there is no amount that is too small to lose but as for me, so far as I have long term hodling targets, i will prefer to move my Bitcoins to self custodial wallets immediately after making the purchase. Exchanges are risky to keep one's bitcoin and leave till you have accumulated enough, don't forget that there were previous cases of exchanges crashing and a lot of people that had assets in those exchanges lost everything so we ought to give our assets maximum protection which is to seek for self custody wallets even though the asset may be very little, it's better to lose part of your assets as transaction fees than lose everything to exchange platforms.
Your idea of wtihdrawing your Bitcoin from an exchange to a self custody wallets is not bad but your reasoning ability looks a bit childish because how can you buy a small unit of Bitcoin and then you immediately withdraw it from an exchange to a self custody wallet? Isn't that stressful like making withdrawals all the time and considering the fact that someone who is using the DCA strategy will need to be buying Bitcoin at regular intervals which means if we are to buy your idea then Everytime we DCA we also withdraw it from the exchange it was bought from and transfer to a non custody wallets. Accumulating upto $500 to $1000 in an exchange isn't that too risky even though the amount may appear risky for you to leave in an exchange but it also depends on the amount you DCA that will tell how long it will take you to accumulate up to $1000 before moving it out of the exchange wallet to a non custody wallets.
Okay just consider the transaction fees it will take each time you try to withdraw a small unit of Bitcoin from your exchange to a non custody wallets, so it looks unprofessional even if a newbie may not understand this but it is still important for them to know because those amount that would be lost as transaction fees can still be retained if you consider accumulating a larger amount before withdrawing out from an exchange.