I do use Gmail, and don't have any but have long planned to move to Outlook.com.. perhaps knowing about this ShadowDOM issue will spur me on to make the move.
Google only has one product that's truly best-of-breed (Maps) and I don't mind using it, but don't want to be entirely in any one vendor's ecosystem. I would say Youtube is the best of its kind, but it's really held up by its community, not functionality as Maps is. Outlook may not be the absolute best for privacy either (it's also no-charge), but it at least gets me to a place where I'm well diversified.
Google Maps, Youtube, InoReader, Outlook, DuckDuckGo all on Firefox with containers is a good enough of a spread for me.
If one cannot avoid it, I think it's a better idea to create Chrome desktop shortcuts for Gmail/YouTube and use Chrome exclusively for that, if you cannot use a desktop email client for Gmail and VLC/mpv/youtube-dl for some reason.