Sometimes I wish I could reset everything without having to carry my laptop over to the router. (E.g. when I mess around with too many settings and make a serious mistake.)
I'm wondering if there's a way to reset everything except the Wifi settings and login credentials (password and/or ssh key).
It seems like this kind of feature should be included in "Backup / Flash Firmware" (/cgi-bin/luci/admin/system/flash). I'd especially want to do this when upgrading to a new release.
Does this already exist, or is there some way to do this in a (relatively) straightforward fashion?
I think the only way to do that would be to build your own image using your own configs, I've never done it myself so can't be of much help unfortunately, but here's the instructions you'll need from the OpenWrt docs (scroll to the top of that page for full instructions on building your own image).
If you're on a linux host and have turned on the ability to log in via ssh you can do a
scp -r root@<IP of your router>:/etc/config .
to save the whole config data to your current local directory. Now feel free to mess around with the box as long and dirty you like...
If something seriously goes wrong and you have left the ssh ability -
scp -r config/* root@<IP of your router>:/etc/config
If you know which file of the configuration is "buggy" you can of course replace only that.
Oh, delete the file "wireless" in the local config directory before copying back the config dir 'cause there are all configurations regarding wifi in, and you mentioned not to alter this. The login credentials aren't stored in one of that files.
Some packages (eg. asterisk) store their config-files not in /etc/config but in a subdirectory of /etc. If you have a backup-file created with luci you can unpack that in an subdirectory of your choice, there you will find all the config subdirectories of all packages you installed - and you only need to copy that file back to your box to get it to that state again.
Now the last thing is to restart the service you're played around with, that it loads the altered config. Hope, that helps a little bit.
The Turris Omnia had an nice feature called Schnapps (https://docs.turris.cz/geek/schnapps/schnapps/). It allowed to store a configuration and roll it back later.
I don't know if something simular exist for Openwrt.