Code
cp /etc/connman/main.conf /storage/.config/connman_main.conf
sed -i s'/PreferredTechnologies = ethernet,wifi,cellular/PreferredTechnologies = wifi,ethernet,cellular/g' /storage/.config/connman_main.conf
reboot
ConnMan "prefers" Ethernet connections by default, so if you have both Ethernet and WiFi active (connected) it will always switch the default route to use Ethernet. If you run the ^ above commands (beware the line-wrap on the second one) this will switch the preference to WiFi so that when both are active the default route will be on WiFi. Is that better?