At the end, after a lot of research and testing I did manage to get my system working.
## my requirement ##
# traffic arriving at a particular interface, fysical or vlan should
# also be answered via *that* interface
## Helpful links ##
# https://cloudinit.readthedocs.io/en/latest/topics/network-config-format-v2.html
# https://linuxconfig.org/netplan-network-configuration-tutorial-for-beginners
# https://netplan.io/examples
# https://netplan.io/reference
# https://manpages.ubuntu.com/manpages/jammy/man5/netplan.5.html helped me
## Issues left ##
# 1) So I did not define a server wide default gateway ....
# .... however that is not true if enp5s0 becomes active ....
# .... I simply do not know how to assign a routing and routing-policy to
# an dhcp provided address
# 2) The trunk is a container for vlans, it does not have an address ...
# .... however I had to give it a dummy address to make this file work
The attached a jaml config file gives being an example for a server having three interfaces. One of the interfaces is the vlan trunk. In the attached file one vlan, but of course it is no problem to add other vlans or to add IPV6 (I assume).
Louis