Thanks - it works, but slightly different from what you recommended. Hope that's fine anyway, I just had a quick look and may double-check if any firewall rule is in conflict with the WAN VLAN (ID=7).
/interface vlan
add comment="Telekom VLAN-Tag" interface=ether1 name=vlan-ppp vlan-id=7
/interface pppoe-client
add disabled=no interface=vlan-ppp max-mru=1492 max-mtu=1492 name=pppoe-out1 \
password=BB use-peer-dns=yes user=AA
/ip firewall nat
add action=masquerade chain=srcnat out-interface=pppoe-out1 src-address=\
192.168.178.0/24
/ip route
add comment="Telekom" distance=1 gateway=pppoe-out1
It works, but only if the PPPoE client ist set to
interface=ether1:
/interface pppoe-client
add disabled=no interface=ether1 max-mru=1492 max-mtu=1492 name=pppoe-out1 \
password=BB use-peer-dns=yes user=AA
/interface list member
add comment=defconf interface=bridge list=LAN
add comment=defconf interface=ether1 list=WAN
add interface=pppoe-out1 list=WAN
/ip firewall nat
add action=masquerade chain=srcnat comment="defconf: masquerade" ipsec-policy=out,none \
out-interface-list=WAN
The bridge was already in the LAN list and now both ether1 and PPPoE are in the WAN list. However, as described, I have to set interface=ether1 in the PPPoE client to be able to login via the Draytek.