I think I got it. It is not Kubuntu but Dolphin that causes the problem. I've tested Nemo and Nautilus and they both allow you to access the "Documents" folder in iOS but for some reason Dolphin doesn't do the job. So I just use one of the former when I'm transferring files through the wire.
You could try like this:
Set your config file in ~/.ssh
Define your host as follows:
Next, in Pantheon Files just set your SSH host as some.mydomain.com with no username and password.
Connect and have fun.
This was resolved in the comments on the question, posting answer here for posterity. The issue was a corrupt apt cache.
The required version of plank comes from the elementary/os-patches repository, which we could see was correctly available on the system from the apt update output.
Despite successful runs of apt update, running apt-cache policy plank was ...