Keeping Mail Boxes Separate

In iOS mail, the “all inboxes” cannot be disabled, but that is merely for display purposes on the device. The mail in that universal inbox still lives in each respective email account inbox - it’s merely displayed on the device in the “all inboxes”. Just scroll down to each individual email account and you can expand each accounts folders and access each account’s discrete inbox anytime.

Your mail accounts are not being moved or stored anywhere together.

Have you thought about separate apps for the different accounts? I do have a mail app called Edison mail, and you can check only one account at a time. Or you could use apple mail for the 2 (let’s just say) that you don’t care about, then a different mail provider for the one you want to keep totally separate.