I've just upgraded by keeping existing settings (sysupgrade without -n param). First upgrade attempt failed, with no message, and router rebooted on previous version 18.06.4. Second attempt went fine.
So far, everything seems fine. I've not tested yet WPA3, and according to https://openwrt.org/docs/techref/targets/ath79 the router is not yet compatible with ath79. My router runs several additional packages but nothing fancy (adblock, ddns, iperf3, email, https, usb drive, a bit complex network configuration with several LANs) plus hardware offload.
root@OpenWrt:~# ping downloads.openwrt.org
PING downloads.openwrt.org (18.104.22.168): 56 data bytes
64 bytes from 22.214.171.124: seq=0 ttl=53 time=26.626 ms
64 bytes from 126.96.36.199: seq=1 ttl=53 time=26.495 ms
64 bytes from 188.8.131.52: seq=2 ttl=53 time=26.501 ms
64 bytes from 184.108.40.206: seq=3 ttl=53 time=26.467 ms
64 bytes from 220.127.116.11: seq=4 ttl=53 time=26.496 ms
64 bytes from 18.104.22.168: seq=5 ttl=53 time=26.518 ms
64 bytes from 22.214.171.124: seq=6 ttl=53 time=26.460 ms
64 bytes from 126.96.36.199: seq=7 ttl=53 time=26.562 ms
--- downloads.openwrt.org ping statistics ---
8 packets transmitted, 8 packets received, 0% packet loss
round-trip min/avg/max = 26.460/26.515/26.626 ms
It's not that hard to flash with sysupgrade from a temporarily (with u-boot option 1) booted initram image.
I think this is the easiest flashing procedure if you have a serial pin header soldered.
Probably you (@liners) can add this info into the wiki page.