I've been using the attended sysupgrade and I guess I'm lucky that there's no migration needed at all. All settings are preserved.
Also, I think the configuration file is already robust enough; you can edit them using a text editor like vim or edit them using the UI or edit them using the CLI tool. I cannot think of anything to make the configuration file even more robust.
I've experienced this on OpenWrt as well. There's a backup/restore config files button but it's sort of a best-effort type of operation. If you are restoring across many versions and the config files are no longer sane, you will end up with a broken system. But the config files were restored exactly as advertised :)
Thankfully all the config files are plain text and can be manipulated however you like. It is usually not too difficult to take working config while fixing or disusing problematic files.
Also, I think the configuration file is already robust enough; you can edit them using a text editor like vim or edit them using the UI or edit them using the CLI tool. I cannot think of anything to make the configuration file even more robust.