So I installed Hubzilla on RPi board with Wifi AP. So it works locally on Wifi and with internet it connects via VPN to web proxy with public DNS records.
Thinking about mobile internet via 3G LTE stick and also battery for operating outside in the country/forest.
But it becomes hard to handle and connect all cables when moving and not so small, so i think isn't it easier to implement it on mobile phone directly, which has everything integrated? Have you tried somebody, looks like only PureOS could have the packages needed? They say Librem5 has the Matrix/Synapse by default, any experience running mobile social server?