How to free some flash memory

See posts above for links about what others have found possible to remove.

I think that installing the packages you need into ROM, rather than post facto onto flash may be sufficient. Certainly worth trying before spending a lot of time removing "stock" packages. Note that ROM packages are compressed, so the size shown in ls or the like won't be the amount of flash recovered (or consumed, if added), but very roughly half of the shown size.

The manifest for an image shows what packages have been installed, such as http://downloads.openwrt.org/releases/18.06.1/targets/lantiq/xrx200/openwrt-18.06.1-lantiq-xrx200.manifest

opkg list-installed will show everything installed on the device. There are some suggestions for scripts that show only post-facto installed packages at Script to list installed packages? (for simplifying sysupgrade)