Why not just add mail address to your regular contacts ? Simpler, safer.

I wouldn’t use a package that does that because it breaks the security of proton, but tbh it’s email who cares.

As far as that package in particular who can say.

From a more 10,000 foot perspective: if you have to ask on a public forum if an application you give access to your secure email account so it can copy and paste contacts into your os address book is safe it’s probably worthwhile to figure out what you’re trying to do and under what constraints.

Leaving the encryption part aside, are you sure that uploading your Proton contacts directly to Google is something you want?

I wouldn't do this. You're leaking important information that was specifically protected by proton to any app that asks for it and to google.

I have no idea about this app.

That said, I use Tuta in part because they allow contact sharing on Android. It may be worth checking whether this app is using a similar approach to Tuta, as I trust that their method is secure.