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Say I have 1000 users, each one with its own account with paths m/49'/0'/{user.id}', and I am controlling received/sent funds.
Could I collect all the funds (i.e. inputs) in a sole fat transaction output even though they belong to different accounts of the same HD Wallet?
note that since you are using hardened keys at the user level public derivation won't be possible. if you instead use non-hardened keys at that level you can store the xpub for m/49'/0'/ on the server and and derive addresses for user level accounts. it's safer than storing the xpriv on a server which you would need to do for hardened derivation. – Abdussamad – 2019-10-22T15:18:28.787
It might be, but that'd deviate from the standard, which would make the wallet much harder to export and manipulate in a different scenario, isn't it? – Molina – 2019-10-22T16:28:41.287