In as much as I like your consistent on Bitcoin investment but one thing you should also consider is the possible fees you could incurred while making a withdrawal on every buy to your wallet, although as investors whose plan is to hold for a very long time it will not be advisable holding your investment on a decentralized exchanges because anything is possible so buying from an exchange and withdrawing to your wallet is actually the best but to be on a safer side in terms the fees of withdrawal you could strategize your withdrawal in such a way that if your accumulation is on a weekly basis you could accumulate for two or three months and withdraw all at once instead withdrawing at every buy which will cost you more.
Keeping Bitcoin on any type of exchange, decentralized or centralized is a big no. We should be wise about where we're keeping our bitcoins. If you have no hardware wallet yet, you should keep it on wallets like Electrum, Bitcoin core, Blue wallet or any of likes that provides your key.
But, wait a minute. Who does keeps his bitcoins on decentralized exchanges? I know that there could be some but really, people still keeping bitcoins there? Because if it's on a DEX, I am sure that it's not a Bitcoin that's on its own network but probably on different chains like wrapped BTC.