Plex does reencode and sync to phone for offline watching. Jellyfin you would stream direct. If you often go places with no Internet then sync is better, otherwise you'd be fine.
You're very welcome. I have a newsletter you can subscribe to if you'd like, though i should probably warn you it costs £5 a month and includes some advertising.
The majority of countries, and the uk for sure, its not illegal to download content but it is illegal to upload it / share it. Therefore using usenet is fine to do, legally, without the aid of a VPN as you are merely consuming and not providing or uploading. I have been doing this for years with zero problem or letter from any of three isps i have been with the in last decade.
Sonarr and Radarr with NZBGet, usenet indexer and a cheap unlimited news server provider, and you're laughing. I have everything downloading automagically as soon as its release, loads right into Jellyfin - all on a copyright notice keen ISP and never have any issues because im not sharing anything up, its all down with usenet
I've never really been much of a TV person, and even then what i do watch is 99% torrented, honestly cant remember the last time i saw an advert so i'll take your word for it :)
If you pre-encode and sync over episodes for later, then yes plex will work better for you. If you watch streaming over 4g then you'd be unaffected