I've been told the AllowedIP setting is for internal WireGuard logic and WireGuard is not controlling the routing table. However, the WG connection is higher in the list of connman services (as it was created most recently) which means it becomes the primary service and default route. To change this we can change the service order via connmanctl, e.g.
connmanctrl move-after vpn_138_116_14_12_wg_domain_tld ethernet_1cf05e0b93f0_cable
Moved vpn_138_116_14_12_wg_domain_tld after ethernet_1cf05e0b93f0_cable
So syntax is: connmanctl move-after <service_to_move> <service_to_move_it_after>
So for scenarios where you want to connect via WireGuard to access content on a remote server, but otherwise still want/need the Internet connection to be as-normal from the local network, we can add another ExecStart in the connman.service file, e.g.
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/connmanctl connect vpn_138_116_14_12_wg_domain_tld
ExecStart=/usr/bin/connmanctl move-after vpn_138_116_14_12_wg_domain_tld ethernet_1cf05e0b93f0_cable
ExecStop=/usr/bin/connmanctl disconnect vpn_138_116_14_12_wg_domain_tld
NB: I haven't tested this, but it makes sense to me, so please have a play and report back.
NOTE: I have subsequently been told that the move-after changes are persistent, so would only need to be done once.