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.