1. Buy a device, https://wiki.postmarketos.org/wiki/Devices#Main . I suggest the PinePhone if you want something with decent power management in the next few months, or PinePhone Pro if you're willing to wait a few months for decent power management.
"Peer-to-peer calls require more bandwidth than calls routed through a server and are thus not suited for a large number of participants. In the future, we plan to also develop calls via a forwarding server to solve resource issues on the client side and to allow for calls with more participants."
Great! Dino is my XMPP client of choice, it's good to see a new release.
My partner and family use https://quicksy.im/ which is an XMPP client that uses their phone number as username, to give it a "convenient as WhatsApp/Signal" feel.
2. Find good programs on https://linmobapps.frama.io/
3. Figure out what functionality you're missing. Contribute improvements.
Keep in touch with the community at https://linmob.net/