The TFTP recovery has worked dozens of times for me during the past 6 years with almost daily R7800 flashing (that goes wrong every now and then). It is pretty bulletproof, as the uboot bootloader is not part of OpenWrt or OEM firmware.
Read the R7800 specific advice from
- Netgear R7800 exploration (IPQ8065, QCA9984) - #5 by hnyman
- https://openwrt.org/toh/netgear/r7800#debricking
One possibility is that you have a bad flash block in the first few blocks of the firmware area. To see that, you need serial cable and serial session during flashing.
In any case, if the TFTP does not seem to work, you best debugging step is serial, so that you see what actually happens. Otherwise it is pure guessing.