Pneumatic Tubes are an option ALONGSIDE belts. Pneumatic tubes dont work on their own and they dont work with bots. They only interact with belts.
The tubes are a 1x1 tile with two pipes - one for each directions.
We have a loading station. It could be a 1x2 tile. When an empty cylindrical container arrives it opens at a holding spot. A belt can lead directly into it, filling the container. No inserter required, items can just fall right into the container.
We also have an unloading station. This one probably has to be 1x1 tile (or a 1x3 long tile: containers would stop in the middle and if a container already sits there, it would go underground underneath and around. otherwise with a 1x1 tile, containers would have to be pushed further if another container bumps into it and takes its place). The container arrives and opens. You can now either take items out of the container (like a chest) with an inserter or you can place a belt in front of it and the items would be unloaded onto the belt.
We also introduce a fan or air compressor entity. It would require electricity and would be required for the system to work. You could stack these entities to allow for larger systems. Each entity would allow for a longer tube length. These entities could have diminishing returns, therefore limiting the tube distance and the pneumatic tube usabiltiy (and wont replace trains for long distance item transportation).
Since pipes cant curve, when a container reaches the end of the tube, it either gets pushed over into the other pipe and send back to the loading stations, or loaded at that side again. You would have to setup fans/compressor entities on each end to allow transportation in each direction.
Here are some pictures for better visualization:
Concept Picture
Thanks for reading. Let me know what you think about this.