Could you maybe elaborate on the feature parity?

1. I travel often. There are a lot of devices in hotels, bnbs, and friend's houses that have native plex support. Not so much for jellyfin.
2. Casting to cast-compatible devices is very hit-or-miss, but mostly miss. I know the casting ecosystem is already a mess, but as far as user experience goes, Plex has spent more effort ironing it out.
3. The native Plex client works with a controller on my bazzite HTPC when launched from the steam ui, while the native jellyfin client doesn't.

I keep trying jellyfin out every few months, but so far keep hitting enough friction that I can't reliably make the switch.

as in separate jellyfin account per each different jellyfin server?

Yes, if me and 5 of my friends have jellyfin servers, we all need accounts on each other's servers. I then need to juggle accounts to access their content.

Jellyswarrm is a reverse proxy plugin I could run to mask the problem for myself, but it's not a solution for mom who may have access to my server, and one other friend's server that I don't know.

The correct solution is federated accounts, but the devs have already stated that they don't want to do that.

Why would you host it openly rather than in a VPN like Tailscale or whatever wire guard is?

Then friends and family have to be on my VPN to stream anything.