Your servers reverse proxy (Caddy, Traefik, Apache, Nginx…) redirects incoming requests to your application (Wordpress). You need to configure the reverse proxy so that it knows which request (origin / the subdomain) to forward to which destination on the server.
Really depends on how often you need to touch your data. Tape has high upfront cost (4-5k $ for a LTO-9 tape drive + ~3,5 $/TB in tapes) but you don’t have to worry about archive space anymore. Otherwise, NAS space (if you selfhost) is ~15 $/TB + a server which would also be slightly above 5k right now to store your 300 TB.