send_payment and send_spontaneous_payment use the same return type Payment as list_payments. They mean very different things, however. list_payments is meant to give you a list of all the balance mutations: receives, sends, closed channels.

Suggestion is to rename the current Payment to PaymentListItem. It will be returned by list_payments.
And use a different model that is more suited for the send_payment and send_spontaneous_payment functions.