diskspace for minecraft server

It will definitely be enough, unless someone purposefully increases the file size over a long time.

My biggest world is my test world, where I have repeatedly flown around for minutes with maximum speed Spectator mode and have teleported around, built self-replicating machines, tried mods and much more. It is about three gigabytes big. Both my Minecraft folders together are less than 6GB.
2B2T is probably the biggest Minecraft world, with 5518 GB (source, sidebar), but they also have had 350677 unique players so far. Assuming this scales linearly (which doesn't quite work, but it's an approximation), the 128 GB in your setup should be enough for your 6 players for the next 1355 years.

What you should really worry about is CPU power. A single Raspberry Pi isn't even able to keep up the tickrate in an empty world with 2 chunks render distance, so you might need a pretty big cluster to host a server for 6 people at once, in a normal world, with proper render distance.