What is the stance on the interest you received from the aforementioned banks, and must it be disbursed all at once, or can it be disbursed gradually, especially since it was received in installments over many long years?

You are not obligated to dispose of the profits you earned from that money, as long as you sought a religious ruling and were given a fatwa (definition) permitting dealings with the bank. If there was any sin, it would be on the one who issued the fatwa to you.

Sheikh Ibn Uthaymin says: If he did not know that this was forbidden, then all that he took is his, and he owes nothing. Or if he was misled by the fatwa of a scholar that it was not forbidden, then he should not give anything away, based on the verse: "So whoever receives an admonition from his Lord and desists, he may have what has already passed, and his affair is left to Allah."

As for the future, you should seek out the best Islamic banks in your country, those with a good reputation for adhering to Sharia (definition) regulations in their financial transactions. You can consult knowledgeable and pious scholars who specialize in banking matters.