but from previous experiences like this, the feature set has to be robust at the start or I think adoption will suffer.

A personalized button gives users a quick indication of the session status, both on Google's side and on your website, before they click the button. This is especially helpful to end users who visit your website only occasionally. They may forget whether an account has been created or not, and in which way. A personalized button reminds them that Sign In With Google has been used before. Thus, it helps to prevent unnecessary duplicate account creation on your website.

They don't look like advertisements. The second the recipient interprets your email as an ad, promotion, or sales pitch—and it does take just a second—its chances of being read or acted upon plummet towards zero. A plain email leads people to start reading it before jumping to conclusions.

Also, if you don't put that implementation of URLSearchParams in the global scope you're not using it as a polyfill but a ponyfill, and those are meant for your code, not for external dependencies.