I bought this AR300M model, with 128MB NAND flash.
I'm trying to build a custom kernel following the instructions here.
However, "Target Profile" only offers AR300M-Lite and AR300M-16 (16MB NOR Flash). No option for AR300M-128.
What options should I pick? I'm guessing I should choose "Generic devices with NAND flash" under "Subtarget", but then what do I pick for "Target Profile"?
19.07.3 does not support nand on the ar300m.
You can flash the nor firmware with 19.07.3 but will have to set bootcount=3 in the existing nand firmware environment otherwise it will boot from the manufacturer's nand firmware rather than your nor firmware.
The bootloader is u-boot and I am pretty sure, resides in the NOR flash. All models of the ar300m have NOR. Some have NAND as well.
Those that have NAND try to boot from NAND first then fall back to NOR.
You can build your own u-boot and flash that, but that is nothing directly to do with OpenWrt and would require detailed knowledge of the flash layout. It has been done - but not by me. Hard bricking is probably quite easy to achieve