Cool, interesting contribution @Charger
One thing I wanted to point is the IPv6 is increasingly important and it is difficult not have it in for a modern broadband connection so wanted to find out if you have tried to include it even removing other parts as usb, pppoe and opkg for example and it it then fits in the 4MB of flash.
Another thing I wanted to find out was how stable it was running18.06 in terms of CPU/Memory.
As a comparison I have tried 18.06 and 17.01 on a older WRT160NL also with a 400 Mhz CPU (older), 32MB RAM and 8MB flash and although they boot up and work fine after some time and access to either LuCI or SSH it starts to become very slow until the point of stopping working including for traffic forward. The version which it became stable again for that case was 15.05.