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.
I disagree with you here. Your opinion might be unpopular because for beginners in investment they have no idea how all of this works. For now they can buy from the CEX and store it in their wallet. All they have to learn is how to buy from the centralized exchange then learn how to secure their private keys so they wont lose their investment. On the other hand i think centralized exchange are more preferable to a decentralized exchange both in terms of buying and storing bitcoin.
Hardly for a beginner to go around using these hardware wallet if they are not much familiar with it talk more of using a decentralized wallet or exchange. They should not attempt it until they have learn how transactions, seed phrases and other basic things. Otherwise they would end up falling for simple scams going on in the internet.
I think we all started from buying from CEX and then to our wallet. So the newbies should not keep that part until they are more acquainted with how the aforementioned criteria.