Sneaky, aren’t they?

Nop. Disable it in the settings/apps

I also don’t have a Gemini app but if you search for Gemini in the Settings main menu, it shows up.

I really want to support this project but my phone is still working perfectly thanks to the Lineage team.

Self hosting an email server is a pain in the ass and I don’t really recommend it.

Buying your own domain and using it in whatever email provider you want is very easy and gives you ownership over the email address. You may switch providers freely without needing anyone else to do anything.