For those who could still be affected by this issue, Cudy have released an OpenWRT firmware for WR1300v1 with support for the XMC flash. I have tested it and it works. It's available at the default download page. By the way, I have uploaded a copy to the space where I shared all the other images previously.
It's seems that currently all WR1300 hw versions supports migration to OpenWRT from stock fw without any hacks.
On top of that, it looks that the have also limit the SPI speed to 10Mhz:
cat /sys/firmware/devicetree/base/palmbus@1e000000/spi@b00/flash@0/spi-max-frequency | hexdump => 0x00989680 => 10MHz
Cudy supported the XMC flash in their OpenWrt images since about a year ago and they never used anything else than 10 MHz in their own firmwares and OpenWrt images as well as in the dts files they published.
So, their images worked with the XMC chip as soon as they added software support for it, but for original OpenWrt this wasn't enough, because also the SPI speed was too high.
Therefore, makes no sense that nobody was able to migrate to OpenWRT just 8 months ago, as it can be deducted from this thread. Moreover, the first openwrt fw from Cudy that supports the XMC flash is dated on november 15th 2022.
I've some images the published on before that date, ex. september, and none of them work.
Any way, I just wanted to notice that it currently works, no matter if it's from yesterday or 1yr ago. Nobody took their time to share that info.