Why must a complete dist-upgrade be done every time?

I live in the middle of nowhere, where the network is slow and unreliable. Why do I have to do a full dist-upgrade? Arch for instance has a command that downloads delta packages when running pacman -Syu and nothing else.

Because every release is a new one, that’s how Tumbleweed rolls… if you want to update then Leap is what should be used.

Leap is a better match for your situation.

From the terminal, start the Update with

sudo zypper dup 

before going to sleep, the next day it will be finished

Yes, good suggestion. Alternatively, try:

sudo zypper dup --download-only

That will download all of the packages. You can then update in the morning, if you like to watch the update while it is going on.

However, I think the earlier advice to try Leap is probably the best idea.