I've been using 17.01.x (1 to 4) on this router over the last year without any issues. It is an old device and has limited flash and RAM (4/32) but for what I use it has been holding up fine.
My custom configuration has no problem persisting over reboots, backups and restores.
Recently I tried 18.06 RC1, the system boots fine and I can access it via the browser, but any saved configuration, even if it's just setting the password is lost after a reboot.
If I revert back to 17.01.4, then the configurations are persistent again.
Thank you, i suppose that should be my next step if I want to use LuCI again, I can disable ppp/pppoe for sure, this router gets the internet by being a WiFi client to an AP and creating a new AP with a different SSID, maybe that will be enough to get LuCI back.
I think the parts that gives you a bunch of free space for other thins will be ppp/pppoe/... and kernel debug stuff.
If you don´t need to install anything after the image is build you could strip down all libraries with CONFIG_USE_MKLIBS=y and strip down the kernel with CONFIG_STRIP_KERNEL_EXPORTS=y.
But bear in mind that this two last steps will probably prevent that you could install packages afterwards...
Edit: Or you could also use the image builder that build only a image without compiling anything and remove here only ppp/pppoe stuff.
But i think the amount of work to build it yourself from the sources or using the image builder is almost the same.
Part of my startup script is installing cURL and the CA bundle to RAM, if disabling ppp/pppoe and kernel debug is enough, I'll be satisfied, or else I would have to bake cURL and CA certs into the image.
-ppp -ppp-mod-pppoe -kmod-ppp
and also in the kernel configuration I removed crashlog, debug info and debug kernel.
# CONFIG_KERNEL_CRASHLOG is not set
# CONFIG_KERNEL_DEBUG_INFO is not set
# CONFIG_KERNEL_DEBUG_KERNEL is not set
# CONFIG_PACKAGE_kmod-ppp is not set
# CONFIG_PACKAGE_ppp is not set