A more mature alternative would is Nextcloud as it offers a lot more, but setup is reportedly more involved. It does appear to be available for enterprise customers as hosted version as well though: https://nextcloud.com/office/
Not exactly free as in free beer but Collabora, and their 'Collabora Online' suite fits your description. It's effectively online hosted libre office with a few extras.
Facebook made a Twitter (now X) clone (Threads) and has reportedly more users than X now [1]. They have also started a Reddit clone as well now (Forum)[2]. Not sure if that one will be a success as well though, as Reddit isn't loosing users like X is.
Reddit was waiting for Digg to have an incident and milked it to billions. Why was there no project waiting for Reddit to have an incident? (It has had many).
It is open source and supported by Nextcloud, IONOS, Proton, Tuta and more.[1]
I haven't tried it out, bu you can find the documentation on how to host it yourself here: https://euro-office.github.io/documentation/
A more mature alternative would is Nextcloud as it offers a lot more, but setup is reportedly more involved. It does appear to be available for enterprise customers as hosted version as well though: https://nextcloud.com/office/
[1] https://github.com/Euro-Office