I have just installed the OpenWrt onto my J4105 Asrock EFI board. I have tried other methods using fdisk to resize partition and resize2fs to resize file system. The steps are many and sometimes not successful. Especially when using the EFI build. I have come across the other method using Gpart. I find it simple and effective. Just make a live Gpart usb and boot up. Follow the GUI to resize the partition. No need to touch anything. Quick and fast.