> ...most critically, its add interface wizard doesn’t set correct settings for a successful connection.

What are those correct settings? Do you need something particularly special done for your Ethernet connection to function? Given that you were there because you were troubleshooting a NetworkManager failure, I'd also wonder if NetworkManager changed in a way that made that "add interface wizard" not work correctly anymore.

> ...the KDE settings UI for NetworkManager is horrifically flaky...

As you've discovered, NetworkManager is intermittently horrifically flaky. Every time I think about how it's the default for most Linux distros, I get so angry.

I used to enthusiastically use it for everything until the maintainers released a version that would always attempt to set the interface MTU to 0 after bringing it up, which the OS would clamp to something else... 12xx or 14xx, I can't remember. [0] After that clownshow, I dropped NetworkManager forever and used the "legacy" tools that had been working reliably for ages and continue to work reliably.

In the case of wired Ethernet interfaces, all that needs to be done for pretty much every home user is to bring the interface up and start a DHCP client like 'dhcpcd'. In the case of WiFi ones, you bring the interface up and start wpa_supplicant which can be managed with something like wpa_gui. Not-coincidentally, the Linux distro I use supports using both of these with its network configuration-and-activation tooling.

[0] For those who don't know what MTU is, know that the effect of this misconfiguration is that your network communications will intermittently stop dead for no clear reason at all. Things have gotten quite a bit better since then, and you can run mixed-MTU networks if you're careful about it... but back then, this immediately launched you into flaky network purgatory.