Hello!
in our setups for now on other vendor switches we can do scenario
when to all vlans witch comes to interface is added QinQ tag and
is possibility to exclude some vlans from adding QinQ tag and pass it as tagged on same port
Is it possible witch mikroltik? watched videos but there is nothing about it.
hello,
hmm… interesting 
i haven’t seriously tried that - but from this wiki
https://wiki.mikrotik.com/Manual:Interface/VLAN#Q-in-Q
I think it should be possible to add non-tagged as tagged on egress interface. maybe can’t be on the first ingress switch (“classifiers switch”) but probably can be tagged on the second one “the writers”.
have a lab. good luck