Thanks @trendy, yes "rather cheap" is right 
Don't mind all the extra GRE tunnel and extra stuff here:
uci export network
package network
config globals 'globals'
option ula_prefix 'x:x:x::/48'
config interface 'loopback'
option ifname 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config interface 'lan'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.16.2'
option netmask '255.255.255.0'
option ip6assign '64'
option igmp_snooping '1'
option stp '1'
option ifname 'eth0 @trunk.1'
option ip6ifaceid 'eui64'
config interface 'cam'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.11.2'
option netmask '255.255.255.0'
option igmp_snooping '1'
option stp '1'
option ifname 'eth0.11 @trunk.11'
config interface 'wan'
option ifname 'eth1'
option proto 'dhcp'
config interface 'wan6'
option ifname 'eth1'
option proto 'dhcpv6'
# used for backup
config interface 'wan_tether'
option ifname 'usb0'
option proto 'dhcp'
config interface 'henet'
option proto '6in4'
option peeraddr '216.66.88.98'
option ip6addr 'xxxx:xxxx:xxxx:xxxx::/64'
option tunnelid 'xxxx'
option username 'xxxx'
option password 'xxxx'
list ip6prefix 'xxxx:xxxx:xxxx:xxxx::/64'
option defaultroute '0'
config interface 'gre'
option proto 'gretap'
option ipaddr '10.99.0.2'
option peeraddr '10.99.0.22'
option tunlink 'wtun'
option network 'trunk'
option df '0'
option mtu '1500'
config interface 'trunk'
option type 'bridge'
option proto 'none'
option auto '1'
option bridge_empty '1'
option delegate '0'
option igmp_snooping '1'
option stp '1'
config interface 'wtun'
option proto 'static'
option ipaddr '10.99.0.2'
option netmask '255.255.255.0'
option delegate '0'
option mtu '2048'
No prefix on wan6:
ifstatus wan6
{
"up": true,
"pending": false,
"available": true,
"autostart": true,
"dynamic": false,
"uptime": 5632,
"l3_device": "eth1",
"proto": "dhcpv6",
"device": "eth1",
"metric": 0,
"dns_metric": 0,
"delegation": true,
"ipv4-address": [],
"ipv6-address": [
{
"address": "2a00:7c40:ffcc:xxxx::xx",
"mask": 128,
"preferred": 2368,
"valid": 3368
}
],
"ipv6-prefix": [],
"ipv6-prefix-assignment": [],
"route": [
{
"target": "::",
"mask": 0,
"nexthop": "fe80::213:5fff:xxxx:xxxx",
"metric": 512,
"valid": 1794,
"source": "2a00:7c40:ffcc:xxxx::xx/128"
}
],
"dns-server": [
"2001:4860:4860::8888",
"2001:4860:4860::8844"
],
"dns-search": [],
"neighbors": [],
"inactive": {
"ipv4-address": [],
"ipv6-address": [],
"route": [],
"dns-server": [],
"dns-search": [],
"neighbors": []
},
"data": {
"passthru": "001700202001486048600000000000000000888820014860486000000000000000008844"
}
}
Compare with ifstatus henet for the HE tunnel:
ifstatus henet
{
"up": true,
"pending": false,
"available": true,
"autostart": true,
"dynamic": false,
"uptime": 8618,
"l3_device": "6in4-henet",
"proto": "6in4",
"updated": [
"addresses",
"routes",
"prefixes"
],
"metric": 0,
"dns_metric": 0,
"delegation": true,
"ipv4-address": [],
"ipv6-address": [
{
"address": "2001:470:1f1c:xxxx::2",
"mask": 64
}
],
"ipv6-prefix": [
{
"address": "2001:470:1f1d:xxxx::",
"mask": 64,
"class": "henet",
"assigned": {
"lan": {
"address": "2001:470:1f1d:xxxx::",
"mask": 64
}
}
}
],
"ipv6-prefix-assignment": [],
"route": [],
"dns-server": [],
"dns-search": [],
"neighbors": [],
"inactive": {
"ipv4-address": [],
"ipv6-address": [],
"route": [
{
"target": "::",
"mask": 0,
"nexthop": "::",
"source": "2001:470:1f1c:xxxx::2/64"
},
{
"target": "::",
"mask": 0,
"nexthop": "::",
"source": "2001:470:1f1d:xxxx::/64"
}
],
"dns-server": [],
"dns-search": [],
"neighbors": []
},
"data": {}
}