Could be a long shot, but you might be able to install a minimal linux distro on it like Arch Linux ARM that already has kernel 6.4 and then run OpenWRT as an LXC container. When you do that, OpenWRT runs off the kernel of the host system. But figuring out how to get a normal distro to boot might be just as much work as getting OpenWRT to build with kernel 6.4, not sure.