Sorry, my mistake - the three routes to the DNS servers’ addresses in my point 2. above must be without the routing-mark=wg-fib.
No idea what I was thinking about, maybe I have copy-pasted too much when copying the gateway name.
Don’t forget to flush the DNS again before testing.
For this kind of discussion, see http://forum.mikrotik.com/t/bobcat-miner-300-relayed/154400/65 .