You just need something that points *.poseidon to your server IP. You can use a host file, local DNS server, or DNSMasq.
Once you have traefik setup, it will have a config where you can setup routers that route the subdomain to a service. Then you have services configured that point to a IP and port.
I based my setup on Techno Tim’s video here and made minor tweaks. Try following that tutorial and see if that gets you started. Feel free to ask any other questions if you run into snags.
I also use a Kobo with Calibre-web and it syncs fine. It’s been about a year so I can’t remember specifics. I remember modifying settings in my Kobo to point to my Calibre-web URL and in Calibre-web settings allow Kobo sync and then make a Kobo shelf. All the books in Kobo shelf are what it syncs too