Installing from the OEM firmware should work ok from the Netgear GUI using the "factory" OpenWrt image (not the sysupgrade image).
(The TFTP approach is a fallback option, which may help, if you run later into problems.)
I suggest that you either install the official OpenWrt firmware 21.02.1 (with GUI), or alternatively my community build, where some defaults settings are a bit tailored for R7800 and the image contains some typical add-on packages.