PrefferedTechnologies should determine which active technology has priority for the default route. If both Ethernet and WiFi have access to the internet then ethernet,wifi should result in the best connection to both internet and NAS. If only WiFi provides access to the internet wifi,ethernet will be required to ensure the default route shifts to wlan0 when the USB WiFi adapter is connected and active. If for some reason traffic to the NAS is routing over WiFi (or perhaps both) interfaces you might need to configure a static route that instructs traffic to the IP of the NAS to route over eth0. There is no way to configure that from ConnMan, but it's easily done using a system.d service, e.g. something like this (untested):
RPi5:~ # cat .config/system.d/static-route.service
[Unit]
Description=Static Route Service
After=network-online.target nss-lookup.target time-sync.target
Wants=network-online.target nss-lookup.target time-sync.target
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/sbin/ip route add 192.168.2.100/32 via 192.168.2.1 dev eth0
[Install]
WantedBy=multi-user.target
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Create the file then systemctl enable /storage/.config/system.d/static-route.service and systemctl start static-route and the connection will be reapplied on each boot.