Plex can be set to auto delete. You can set it to delete after something has been watched (after a delay if you want), or to keep a pre-set number of items (e.g. only keep the five latest episodes of show X) or a combination.
Just make sure you set up the *arrs to not re-download the thing that Plex auto deletes.
I’m a hoarder so I keep a lot, but anything that’s time-sensitive like current affairs shows, I delete after watch and set to only keep the latest three episodes.
There’s an article on Apple Support about importing mail here: https://support.apple.com/en-gb/guide/icloud/mmf9877972f7/1.0/icloud/1.0
I think this is a Thunderbird issue. They are separate in Mail.app. It even allows you to use a catch-all rule if you’re the domain owner. Maybe you could set up smart mailboxes (or whatever the Thunderbird equivalent is, I’ve not used it in probably 15 years)?