TempleOS

You could use Syncthing for local folder syncing between devices. It’s been really reliable for me.

Well, I completely understand your point. I also have rechargable AA batteries laying around, but I see it from this standpoint:

1. The Steam controller includes this magnetic charging puck, which is a good way to make sure it’s always charged.
2. It will mostly be used more or less stationary, sitting down in front of a PC. So even if the need arises to charge it this shouldn’t be a problem in many situations. This is vastly different than with the new VR controllers, because they will be moved around a lot and it’s not really realistic to charge them while using them.
3. Many people still just use disposable batteries. Which is quite frankly just not good. So my best guess is that this connected with point two may be a leading factor for why they did it this way. I think a hybrid option (puck-rechargable battery pack or two AA’s) would have been awesome though.
4. Valve’s repair policy was exceptional with the steam deck so I have reasons to believe that the battery for the steam controller will be available for a very reasonable cost. A very interesting question will be for how long.

All in all I understand your point, but it’s not a huge issue for me personally.

The siblings could run
tailscale ping nodeIP/nodeName
This not only shows if they are connected via a DERP (relay) but also retries to connect them directly a few times.
If that doesn’t work out it can sadly be quite hard figuring out why a DERP is used instead of a direct connection.