sudo zypper packages --unneded will give you a list of packages that have not been explicitly requested and are not dependencies of explicitly requested packages. As for how to remove them… IDK (I do it manually, once in a blue moon: it’s not like there’s new unneded packages every week).
It’s been a while since I’ve used debian, but IIRC apt autoremove will leave behind config files (unless you specify --purge).
In tumbleweed (and I think all rpm-based distros?) config files are removed per default together with packages (well, the config files installed with the package, not others that may have been created later such as the ones in your \~ - basically zypper rm is the same as apt purge).
Configure it like the current router and keep it as a backup?
You can run a lot of stuff on it, but those boxes aren’t really that powerful… a cheap, old raspberry pi from ebay (or anything really) will serve you better.