If you import email via any number of means into gmail or app in any way, you're feeding the beast despite what they say. You can configure gmail not to scrape users from email contacts, and I do this default, but doesn't mean they're not in their database still for "private" use by google.
Agree on k-9 mail app with non-google email services (prior godaddy), or the commercial aquamail to replace k-9 as a better option, both keep things separated well enough, though who knows what google does with that app data still however internal to the phone. I moved to google domains/gsuite email after my last work stint at godaddy, and find things are much better despite selling my soul entirely to them. Usually, this week other reasons have me hating google.
There is little privacy left, and google's long known everything about me anyways, so I'll trust them over say, microsoft o365 or apple services.