Yeah, xmpp is a great protocol, but everyone needs to be in the vicinity of the server. A mesh network (p2p) where every client also relays messages for other clients is better suited for this usecase. I think meshtastic is an open standard for this. Briar is a valid alternative that works with phones, which most people already own. Not the greatest idea to bring those to protests, though because of network surveillance attacks by police forces. A meshtastic device would solve this problem.

Consider undervolting cpu and gpu and you will be able to get away with a leaner psu. I always do this and there really is no downside if you overclock the gpu a little to compensate for the undervolting. Your system will run cooler as well often stopping cpu/gpu from throttling down, so with a bit of luck you can even gain performance while saving on power. It depends on the cpu/gpu combo, but you can expect to save around 50 Watts under full load with both components properly undervoltet.

Here’s a very good guide for undervolting your gpu under windows, it’s meant for nvidia gpus but the process is very similar with amd gpu’s and/or under linux.

https://github.com/LunarPSD/NvidiaOverclocking/blob/main/Nvidia Overclocking.md