There is a software that has a technopolitical project behind it called Decidim. It comes from the legacy of 15M - 2011 in Spain, where the organisers needed an alternative like the one you mention, and to have a ‘facebook of democracy’: https://tecnopolitica.net/en/content/white-paper-decidim. The Barcelona City Council made the project possible and now it has an international community with more than 400 organisations, including many local communities. Apart from being an open source and democratic project, it is a very mature product that has not lost the orientation of the spirit of its creation.

Decidim is a political social network that allows communities to have a free technology, with democratic guarantees and designed for the common good. While this technology can be installed with knowledge of Ruby on Rails and some knowledge of servers, so perfectly self-hosted, there are also organisations that offer it in SaaS format at a very competitive price. Also, you can federated differents Decidims:)