Actually AIUI iptables command line tool is not necessary for OpenWrt firewall configuration to work. OpenWrt fw3 uses the userspace libraries that iptables cli also uses to manipulate the kernel firewall, from what I understand. So the firewall is actually still there in OpenWrt with only luci-app-firewall and not requiring the iptables and iptables6 packages. I wonder oi @jow could confirm?
Correct, neither iptables nor ip6tables is required for firewall to work. However when you remove IPv6 related firewall components you should also embed a custom sysctl snippet disabling IPv6 completely to avoid accidentially exposing WAN to the IPv6 internet.
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.