Harry Potter characters is a perfectly reasonable server naming scheme. Server names should be easily recognisable but not tied to any particular service/project/function on that machine (as the server may be used for other things later etc)

See RFC 1178: https://www.rfc-editor.org/rfc/rfc1178

Speaking from experience, I’d recommend not choosing protonmail because it is very difficult to cleanly stop paying and leave - forwarding mail to another address is a paid feature. It’s also not very standards compliant (no IMAP/SMTP support without running a buggy bridge). Their security features are mostly useless and better security can be achieved on standards-compliant services if you try.