I have no knowledge about flashing from Voxel firmware. No idea if that accepts OpenWrt images.
But the tftp recovery flash method (in u-boot bootloader) always works. It requires an OpenWrt factory image. Read instructions in r7800 OpenWrt wiki and e.g. in forum
You should either take my newest 23.05 build factory image (that has support for USB storage, SQM etc. built-in) or the official 23.05.0 factory image (into which you might need to install a few add-ons).