R7800 is otherwise easy, but like said above, in the initial install from the OEM firmware you need to trigger the TFTP recovery flash mode at boot. But even then you can use TFTP tools with GUI from Windows, e.t. tftpd64. The TFTP mode is well documented and works well:
- Netgear R7800 exploration (IPQ8065, QCA9984) - #5 by hnyman
- https://openwrt.org/toh/netgear/r7800 (Debricking section)
After the initial install, sysupgrading R7800 in master and 18.06 builds works ok from the Openwrt GUI.
WRT3200ACM also works ok and flashing is easy, like said. And there is the dual firmware mode for fallback. (In case you want to return to the OEM firmware, you can flash that with sysupgrade --force option from the Openwrt command line.)