Rain

You could host a mumble server on a LAN

Do you have any photos of this?
Would love to see how this looks in practice!

Elements (Matrix) as much as much as possible. I bridge a lot services through it. Though Signal would be the second most.

Was a big fan of RedReader.

No as that hardware would be way under powered for almost any of those tasks on their own, let alone for all of that on one machine. Your best bet would be to look at the hardware recommendations for each service you are interested in and go from there.