This may be a naive question but here goes.

Im building a way for users to use fiat to buy Bitcoin - but not sell it (they can at a later stage withdraw it). Basically like a direct debit savings account.

Users create an account, go through KYC, and add funds via a debit card.

When a user creates an account they provide a seed phrase, and this can be used to generate a hierarchical deterministic (HD) wallet.

I then will acquire Bitcoin for the user (not sure how this part will be done yet), generate a public key from the HD wallet, and send the Bitcoin to this. I wont save the private key or the seed phrase since I can later, if they would like to withdraw, generate the private keys for the users wallet from the seed phrase they provide (as far as I understand).



Is this plan feasible and is it secure? To me, it looks secure since Im not storing private keys.