Being a wise and thoughtful business owner, you already ran this concept by your own corporate attorney to get a written legal opinion before circumventing their TOS, right? ... No??

IMHO, stop calling yourself a Midjourney API of any kind and stop providing a service that uses Midjourney as a backend. You are not authorized to run Midjourney jobs using other people's account credentials; that's the TOS violation. Your lawyer will explain this to you.