I got it to work some times ago. All I had to do was add a combination of lines to /etc/pf.conf starting with nat (to indicate that openbsd acts as a NAT router) and pass (to allow packets through the firewall), and a dedicated line for DNS requests (the examples in Combination of OpenBSD sys-net + MirageFW - #12 by nalea are for HardenedBSD, but the syntax for OpenBSD isn’t so far).
Unfortunately, I won’t have access to that laptop this week and won’t be able to help you further for the time being.
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