In this case you'll have to go with all ports part of single bridge. ether1 will be trunk port for VLANs 10 and 20. The rest of ether ports will be either trunk ports for VLAN 20 (the IPTV ports) or hybrid ports (untagged for LAN, let's say LAN will be VLAN 30, and taged for VLAN 20) - for IPTV boxes that might need access to LAN for some reason - or simple access ports for LAN (for LAN machines). You'll have two vlan interfaces: WAN with VID 10 and LAN with VID 30. No need for bridge to have anything above L2 for VLAN 20.
E.g.
/interface bridge
add bridge frame-types=admit-only-vlan-taggedvlan-filtering=yes
/interface bridge port
add bridge=bridge interface=ether1 frame-types=admit-only-vlan-tagged ingress-filtering=yes # pyhsical WAN port
add bridge=bridge interface=ether2 frame-types=admit-only-vlan-tagged ingress-filtering=yes # IPTV port with VID 20 tagged if that's the way set-top box requires it
add bridge=bridge interface=ether3 pvid=20 frame-types=admit-only-untagged-and-priority-tagged ingress-filtering=yes # IPTV access port if IPTV set-top box expects it untagged
add bridge=bridge interface=ether4 pvid=30 frame-ftype=admit-all ingress-filtering=yes # hybrid port - untagged LAN and tagged IPTV
add brdige=bridge interface=ether5 pvid=30 frame-types=admit-only-untagged-and-priority-tagged ingress-filtering=yes # LAN untagged only
/interface bridge vlan
add bridge=bridge tagged=bridge,ether1 vlan-ids=10
add bridge=bridge tagged=ether1,ether2 untagged=ether3,ether4 vlan-ids=20
add bridge=bridge tagged=bridge,ether3 untagged=ether4,ether5 vlan-ids=30
#
# Note: in /interface bridge vlan specifying ports as untagged is entirely optional as ROS will add relevant ports as they have pvid setting under /interface bridge port
#
/interface vlan
add interface=bridge name=WAN-v10 vlan-id=10
add interface=bridge name=LAN-v30 vlan-id=30
#
/interface list
add name=LAN
add name=WAN
/interface list member
add interface=WAN-10 list=WAN
add interface=LAN-10 list=LAN
....
/ip dhcp-client
add interface=WAN-v10
/ip address
add interface=LAN-v30 address=<LAN IP>/24
...
etc
It is extremely important that you get the L2 setup (bridge and LANs) in this case right ... The way your ports ether2-X will be configured highly depend on how IPTV set-top boxes expect things to work. In my case ISP delivers IPTV multicasts tagged (and set-top boxes require it tagged with same VID), but set-top boxes also want to have normal internet connectivity untagged (I created a separate LAN subnet for them) ... and normal internet connectivity is used by them to download EPG and to access some 3rd party services (youtube, deezer, ...). They can also play contents served by local DLNA servers but I don't want that set-top box to have anything to do with my LAN.