You must use TRX-image for AX59U router!
Also look this: Asus TUF AX4200 support - #223 by remittor
Similar.
No. This is a special initramfs image that is inside a special TRX-container.
I tried to add support for generating TRX-images to the official repository, but I encountered very strange behavior from the OpenWRT maintainers.
Therefore, most likely there will be no official support for TRX-images 