oui well, stupid me shouldn't be doing these things without full attention. You can't zcat an image that you already gunzipped (and why would you do that anyway?) 🤪
So, it just works, if you do the common sense thing. I managed to get it to boot and resize the root, and connect to my WiFi, headless. But it must not be booting fully: ssh connection is refused (sshd is enabled in rc.conf) and telnet connects and quits right away (it is enabled in inetd.conf). Oh well nothing I shouldn't be able to figure out.
I'll probably switch to 10 either when I get this thing fully configured. (I've got five other machines running 9.3, and they're all going to 10 at the same time). Or, more likely, when 10.0 is released.