Don’t know what users here think about posting links to one’s own site, but I’ll risk it. I migrated from gmail to proton, and wrote down the steps and experience. [1]

Should be the same for tutanota or others, don’t know about migration script (easy switch) of others, though. For me it worked without any hiccups.

[1] https://blog.metters.dev/posts/leaving-gmail-behind/