Thanks @slh for the links.
I already found the IPQ40xx Switch Config Strangeness topic, but didn't understand the restrictions of this "strangeness". Moreover, from IPQ40xx: Is it possible to use vlans? I was not sure if there is a working solution for eth1.
What I can confirm, is that with OpenWrt 21.02.1 it is possible to configure VLAN on eth0/switch0. I successfully tested a configuration with 3 VLAN IDs, running untagged on 3 ports and tagged on the fourth port of switch0 in the EA8300. So I wondered, if some kind of driver level VLAN configuration would already allow to configure a VLAN on eth1.
The PR ipq40xx: introduce proper ethernet and DSA support doesn't mention the EA8300, so I'm not sure if it would address my specific problem.