I installed and configured the target 18.06.8 on a TEW-714tru (8MB/32MB single-port device). I didn't want to go to 19.07.2 as probably requires more resources.
Currently it shows almost 4MB free of flash (I think that should be OK?), and around 5MB free of RAM (excluding buffers). But opkg update runs out of memory, therefore I cannot install luci-app-sqm and/or other packages.
I'm aware I could build my own image substituting things I'm not using, like ipv6, but for a novell OpenWRT ubuntu user seems not a trivial task to accomplish without reading a lot first. As a test, is it possible to remove unused modules on the running image, to check the impact on the free RAM?
As another workaround, I looked for luci-app-sqm and its dependencies to manually install it from my PC, but I didn't see those packages on the Release Downloads folder. Are the proper .ipk publicly available?
After been pointed in the right direction and walked through download and copy all packages dependences, I installed the ipk I wanted. It works, but I notice the free RAM dropped to a 3MB, so if I like the package behaviour, I will build my own target with some modules removed, aiming to save some additional RAM.