Buying bitcoins little by little and withdrawing them to a personal wallet is the most effective way. I've been doing it for a few months and now I think I'm pretty successful even though I only have a few bitcoins at the moment. But I hesitate to buy again when the price of bitcoin becomes too expensive.
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.