I don't know you, your daughters, or their friends so I can't make specific recommendations. What I can say is that it's really common for teenagers who are sheltered from the dangers of the world to make more and bigger mistakes once they're unsupervised than those who get a gradual introduction.

The two main dangers of social media for most people are:

1. Encountering assholes. For girls and women, there's a high probability assholes will try to sexually exploit them. Since there are minimal consequences most of the time for sending "show me your tits", they're going to encounter that behavior eventually, and it may be easier to deal with for the first time when they have parental support.
2. Algorithmic rabbit holes. These can create the perception that problematic attitudes and behaviors are common and widely accepted when they are not. Having an open dialog with parents about anything from eating laundry detergent to Jordan Peterson can be a strong stabilizing influence.

I don't think a closed Fediverse server is likely to serve as a first step in a gentle introduction because it has neither danger and presumably no strangers to talk to. The full Fediverse might work better, as it does offer interaction with strangers. Encounters with assholes will be less frequent than on corporate social media, and any rabbit holes will be much more self-directed.

That said, when one of them is likely within a year or two of leaving home or at least having full control of her digital life, if she wants to use some corporate social media, she's probably better off doing that with some parental supervision and support than jumping in completely unprepared when you're no longer in a position to prevent it.

Her friend group has a group text and she wants to keep up with everyone but doesn’t want to get the ding notifications constantly.

This seems like a good opportunity to learn how the notification settings on her phone work.