If I set up, for example, 30 slots and deactivate 29 so that only one is active and the inventory is sent, all slots are activated again.

The goal is to have only 1 slot and teleportation to occur after filling the inventory with only 1 slot. I disabled the rest of the slots.

Fixed!

Nope.. partially fixed.
If the teleporter has quality and therefore a larger inventory, it also disables slots that should not be disabled. It behaves as if it only activates those according to the settings
Try increasing the teleporter quality and try it

If one teleporter is a different quality than the other, the position of the red bar won't look right when it's copied from one to the other because it's just a number that represents an offset. Between this and the synchronization issue, I should probably just disallow teleportation between teleporters of different qualities.

And wouldn't it be easier to just remove the items from the input inventory and move them to the target inventory in another entity? There would be no problem with disabling inventory slots and the GUI wouldn't flicker when teleporting

Quality shouldn't affect functionality. I understand the speed of teleportation, energy consumption, but changing the inventory configuration?

It now disallows teleporters of different quality by default, but you can disable this if you really want to. Whether to swap the inventory configuration between teleporters is now also an option.

I don't know if that's a wise solution. Wasn't it enough to just check if the target inventory is big enough for all the items? If so, then teleport and if not, then a message that the target inventory is not big enough?

But at least you added the config ;)