Without having checked it out properly outside of the landing page, here's a brain-dump of things that competitors get wrong and you can improve by a lot: 1. Support self-custodied 2FA: TOTP/U2F. Do not require phone-number/SMS "for your own protection". 2. Be honest with your customers and users. Don't succumb to dark UX for the sake of control, engagement and growth. Allow setting separate notification settings for…
A lot of people, in the tens to hundreds of millions, not through choice of not wanting to use Google Play Services (or knowing what it is) cannot access Play Services on their locally purchased / locked device.
These people are largely outside North America / Western Europe, potential customers for whom a service such as that of OP may be very attractive.