One of the differences between those devices, mobile phones and other small units vs a "normal computer" is the concept of System On Chip (SOC) and Flash ROM. Flash ROMs are usually very limited in storage size and the whole design is what makes such devices both cheap and low power consumers. The flip side is that unlike normal computers, the image flashed to the device ROM must be tailor-made for that specific device in a manner that will fit in the ROM capacity of the device. (I'll leave it to you to look up SOC, embedded systems & ROM Flashing).
To answer your question, you would be extremely ill-advised to attempt a flash to an SOC device any OS designed for a computer. You would have to a) compile the OS for that specific chipset (arm for example) and b) figure out all the drivers needed to packaged c) create a raw disk image offset so as not to over write the device bootloader or provide a compatible bootloader in the image file.
OpenWRT is a bit complicated at first, but you just have to play around as with any software. I found the firewall the most unpleasent, but not much other choice out there. There is the FreeBSD Open Router project (
https://bsdrp.net/) but has a limited number of flashable devices in inventory as I recall. Other open projects exists, you'll just have to search for the one that best suits you.