If this is a concern of yours, don't migrate your account. All instance admins play the role of Twitter CEO on Mastodon, which means (much like Amazon, Apple, Google, Facebook, Microsoft, Netflix, et. al) they can access all data you've trusted them with. The point of Mastodon is that it broke down these data silos, and give people more sane ownership models for social media. Your privacy concern is valid, but Mastodon doesn't advertise itself as a private protocol. A glorified microblogging platform doesn't really have a whole lot of data to leak besides maybe your DMs.
> We should have the expectation for private data that the operator cannot read it
That's called heterogenous encryption, and it's the technological equivalent of Mythril. End-to-end encryption doesn't stop the operator from decrypting your data. In fact, pretty much everyone has to, since raw encrypted TLS data can't just get slotted into your OneDrive/iCloud account. These operators literally need to read your data to operate on it. I genuinely don't know how you would engineer a more secure architecture here.
If you want to talk about architectures designed for malicious actors, you probably shouldn't start with distributed systems. Monolithic, profit-driven corporations like Twitter are much easier to tempt with salacious "data brokers, political shadow companies and "the feds""
Having publicly readable posts is core to the whole idea, just like Twitter.
Note: there are some interesting forks like Hometown[1] that have interesting privacy variants. The big feature I'm envious of in Hometown is the ability to send a message _just_ to people on your server that will never leave it. BUT overall mastodon is 100% about publicly readable information (like Twitter). If someone isn't comfortable with that they shouldn't use Mastodon.