Finally, tried to manage packages to fit in 4MiB. After removing some packages, custom build firmware worked fine with V-3 hardware.
For those who want to try, I build two versions. One for standalone and another for extroot support. Both versions removed ipv6 and pppoe supports.
1) This version supports
* 3G, usb, and
* ext-4 file system. Like @bobafetthotmail said, this version can detect SD card inside 3G-dongle.
* And I added mwan+luci-app-mwan3 for multi-wan support. Tested this also worked (Failed-over worked, load balancing is not tested)
Leaving about 104k space after flashed. Good point is you can still use extroot with this build (with ext-4, no vfat)
2) This version-2 is especially for extroot
* usb, and ext-4 and VFAT file system. This version can also detect SD card inside 3G-dongle.
* No mwan3 and no serial, no 3G dongle support. You can install whatever after extroot
Leaving about 76k space after flashed.
By the way, Dear LEDE admin and developers, LEDE aimed for flash at least 8MiB, right!. But the problem is these cheap routers with USB support only have mostly 4MiB. Pls also consider for devices with 4M flash for some transition time.
I like this 3420 router because this one is dead cheap and full featured. Some alternative like 3020 and 703 are also cheap but very small and no feature like eth-switch and little gpio.