It's at least seamless by installing a single app, Davx5. Alternate Android distros like Calyxos come with Davx5 pre-installed, so it might as well be built in there.
Yeah, though I've found with Pixel software, it's still a bit annoying because Contact/Calendar apps will always show your Google account first, then keep prompting you to turn sync back on if you turn it off, etc. Flash your own ROM and you get a perfect experience there, but you get a whole new set of problems. These tradeoffs are a lot of why I moved to an iPhone this month for the first time since the 4S. Say what you will, but iOS integrates with Fastmail like it was built for it.