mu4e+mbsync+msmtp
sylphio
The ancestral high-protein diet: drinking the tears of your fallen enemies.
The Malazan Book of the Fallen, by Steven Erikson.
That could be very useful when using a text-based web browser (e.g. TUI)
I am not an expert, but I see that the Apache license has no copyleft.
I think Matrix has bridges to all these networks, except ActivityPub (it may, I just don't know).
I don't know about OP, but it is available in https://thecapslock.gitlab.io/fdroid-patched-apps/fdroid/repo and https://calyxos.gitlab.io/calyx-fdroid-repo/fdroid/repo
I have seen many different ways to achieve that in Emacs. Do you have one to recommend (modes, packages)? Do you synchronise calendars with other devices (e.g. smartphone, server)?
https://github.com/spantaleev/matrix-docker-ansible-deploy makes it easy to deploy.
Not sure about the RAM limitation. If Synapse is too heavy, perhaps a different server (Dendrite?) will be light enough.
https://pubmed.ncbi.nlm.nih.gov/8262106/