more than 16mb

https://forum.archive.openwrt.org/viewtopic.php?id=43237&p=19#p259119

I'm game for this.  Does anyone know where to buy a W25Q256 chip?  They are "non stock" at Digi-Key.

The 32 MB chips use a different SPI command access the top 16 MB.  This is because the standard SPI commands for 16MB and smaller chips only use three address bytes, which is only sufficient to address a 16 MB space (0x1000000 = 16Mi).  The W25Q256 includes a compatibility mode where the first 16 MB can be read/written using the old commands, and a page selecting scheme where 16 MB blocks of the chip can be swapped into the 24-bit address space.  Probably the kernel is using page selection, since the hardware SPI flash controller is likely not aware of the 32-bit addressing mode.

But all that you really need the bootloader to do is access the first 4MB, which the stock one should do in compatibilty mode.  As long as your kernel is in that space, it should be able to boot it, then the kernel will take over and be able to access the whole chip.  To get the ART in place, and/or to flash an image that is larger than 4 MB, you can load and boot a RAM-based OpenWrt which has ART writeable.