You are not unaware of ProtonMail [1]. Why are you putting its name in sneer quotes to imply unfamiliarity and insignificance?
SEO it is hard to say. They want to compete on "email" and "secure email" keywords it is not going be easy.
G Suite Email is a paid service that doesn't sell your privacy, doesn't serve ads, and has extremely good service.
G Suite is an american service and will give away your privacy without a proper/open court case when receiving a gag order. This is very much in contrary to PM.