For event management, Framasoft created Mobilizon as a free and federated alternative for Facebook events.

From their blog article announcing this development:

From climate walks on Facebook to free software hackathons using Meetup : to change the world, utopians (like us !) too often organize themselves on the centralized platforms of web giants.

At Framasoft, we thought it was important to take the time to think about an alternative that could change the situation. We have just spent a few months, with the help of two designers (Marie-Cécile Paccard and Geoffrey Dorne) who haved interviewed many activists so as to better understand their digital practices. We looked at what a tool that would really empower individuals and groups could look like.

For project management, there is System-D.

I think both are directed to activists. They just don’t have the marketing budgets of centralized web giants so their reach is less important.

For messaging, several alternatives exist. https://gofoss.net/intro-speak-freely/

I also think that a lot of activists sadly do not understand privacy issues. They use Facebook as everybody else just because it works and they don’t have to pay money for it. If they don’t understand the issue, they will not search for a better tool.

E-mail providers could configure another character for subaddressing, broadly accepted in forms, but they stick to this plus sign sadly.

I think about trying to buy a domain name, and playing with catch-all and ignore rules to avoid spam while creating a custom subaddressing setup on a custom domain. Source: this Mastodon post https://chaos.social/@silmaril/108878671259313909

Your comment reminds me the PeerTube situation. Only one developer was needed to create this YouTube alternative.

The article mentions it will be better later this year:

Nextcloud Social is available as an alpha version for earlier releases, with an update to Hub 3 coming soon. The team at Nextcloud is working with the community to add new features and give users a way out of the Big Tech sandbox. We will make the updated Nextcloud Social a default part of Nextcloud later this year.

They did an overhaul specifically because of Musk according to their blog:

That is why we began working on Nextcloud Social, to make sure there would be a credible alternative by the time the deal closed.

https://nextcloud.com/blog/no-need-to-trust-musk-we-go-federated-with-nextcloud-social/

You can see and comment Lemmy posts from Mastodon.

Well if he could increase the power a little bit in winter and decrease it a little bit in summer that would be better! 😃

Maybe you can find something in this list? https://www.gnu.org/distros/free-distros.en.html