Using gmail has caused me no negative effects. It may not be ideal, but it works and has a reasonable set of tradeoffs.
I think it's more like renting vs buying or building from source vs compiled binaries than it is a crippling substance addiction which will ruin your health and relationships.
I'm using Office 365 myself, because I hate google's datamining but Microsoft isn't much better. The added benefit for me is that I work with M365 at work and having an unrestricted tenant for myself helps me understand it better (to see the options available that I don't have access to in the admin console at work).
But I don't know what else I'd use. I don't like proton very much as a company either (purely personal opinion), and I don't know anyone else that has good mobile syncing.