I wonder if they'd like a percentage of the profit now and in the future? Your reaction to the C&D shows some resilience, which is nice to see.
I used to worry that the biggest threat was them releasing an API.
I’ve since learned that they won’t do that because they charge monthly fees to their customers and when a customer doesn’t use their credits, the credits don’t roll over and Midjourney pockets the cash.
Pay per use is bad for their business model.
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.