that's why i'd suggest to change grub to use simple partitions:
menuentry "OpenWrt-21.02.3-sda2" {
linux /boot/vmlinuz root=/dev/sda2 rootwait console=tty0 console=ttyS0,115200n8 noinitrd
}
PARTUUID in frequently changing hardware environment might make sense but it is a router, once it is up i don't want to add disks which might change disk order. so personally the very first thing i do is replacing PARTUUID. then can resize/add partitions as needed without jeopardizing hanging during boot because UUID i forgot to update (e.g. after partition resize).