root@OpenWrt:~# mount
proc on /proc type proc (ro,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
overlayfs:/overlay on / type overlay (ro,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
tmpfs on /dev type tmpfs (ro,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (ro,relatime,mode=600,ptmxmode=000)
root@OpenWrt:~# df -h
Filesystem Size Used Available Use% Mounted on
tmpfs 929.3M 10.6M 918.6M 1% /tmp
overlayfs:/overlay 92.5M 20.5M 65.0M 24% /
tmpfs 512.0K 0 512.0K 0% /dev
squashfs is indeed RO, but that is the purpose of the overlay -- the overlay should mount as rw and will normally contain all files changed or added by the user (including config files).
Is it an option for you to run the standard release build? 19.07.5 is the latest.
if you are using a snapshot image, and the snapshot packages are missing in current builds.
I don't believe that this is an issue at the moment.
But yes, building your own image makes this easy -- you can include all the packages you need at the outset.