I realize that, but the r7800 is a rather popular and well supported device for OpenWrt, which is relatively hard to brick thanks to a robust tftp push-button recovery mechanism. You can flash OpenWrt using that procedure (mandatory, don't try to flash OpenWrt via the webinterface) and revert back to the OEM firmware the same way, if you happen not to like OpenWrt on it.
18.06.2 will run fine on it, but there are (optionally) two community builds more targetted at this particular device:
- Build for Netgear R7800
- Optimized build for the TP-Link C2600 / Netgear R7x00 / Linksys EA8500 / Zyxel Armor Z2
Disclaimer: I'm using the very similar ZyXEL NBG6817 myself, which is working very well.