I am really starting to love you

Allow me a simple consideration about alts being used for transport:
They are used because the only alternative is transport by feet (or other means that are mostly comparable). So teleporting alts is the preferred solution.
You don't need to physically eliminate the possibility for alts, you just give a more engaging, fun alternative.
If you could build a wagon which holds so many goods that an alt could only possibly transfer in 4 hours of boringly repetitive teleportation... I bet people would start organizing and defending wagon trails.
You fear that is not enough? Consider introducing a timer for teleporting that scales with the quantity of items you have in the inventory.
Now there is also a reason for road building, inns and outpost.
Oh boy, that would be sooo more fun.
Also consider rafts, barges and ships.
Water transport has always been a much more preferred solution over land transport.
...I know, that would also require more advanced containers to be accessible in claims, bases. Maybe depots that can hold a lot of items but only if they are of the same type?