Not sure if I entirely understand what you’re asking but here’s my setup that sounds similar-ish that might help.

I’ve got essentially 3 machines

1. Download machine - contains Sonarr/Radar/Nzbget, etc… This machine isn’t very powerful but it has A LOT of RAM.
2. A Nas - this is where everything gets downloaded to. Primarily this machine just has a lot of HDD space.
3. Jellyfin box – Decent RAM and a beefy CPU for transcoding.

The download machine has a network share to download directly to the NAS in a special /downloads/ folder. Once a download completes Sonarr, etc… move it to it’s correct media folder.

Finally the Jellyfin machine is monitoring the media folders for changes.

I assume you could set up something similar with Plex instead of jellyfin and then store the fully downloaded files on a separate machine with a network drive, so Plex can see it. Essentially the NAS for you would be two machines one (the seedbox) for the partial downloads and a local NAS for the fully downloaded files?

Anyway, not sure if that’s what you’re looking for.

I believe what you’re looking for is Remote Path Mapping, read this guide and see if it fits your situation - https://trash-guides.info/Radarr/Radarr-remote-path-mapping/

Sonarr should be the same setup process. Essentially, you want your arr’s to be able to see and access the files on your seedbox. Best way that I’ve found to do this is sshfs or rclone’s sftp. Mount your seedbox’s download folder on your Plex server with sshfs/sftp

Since you’re also moving the files from your seedbox to your Plex server with synching, I imagine a setup involving mergerfs will work too. Basically, you mount your seedbox download folder to your Plex server, use mergerfs to merge it with your local Plex server’s location that Sonarr/Radarr imports it to - point Sonarr, Radarr and the Plex to the merged location. When the files are being moved by Syncthing, all your services will still think they’re in the same location so it doesn’t get confused.

If you want to go that route, you can use the Google Drive guide - https://web.archive.org/web/20220508210909/https://wikiold.servarr.com/Cloud_Setup and change it a bit so it works for your configuration. Ignore the parts about encrypting and the nightly move script, Syncthing does that for you. And instead of mounting a Google drive remote, you will be mounting your seedbox through sshfs/sftp

I have a similar setup as you so maybe I can help. BUT I don’t get your question.

Anyway, my setup:

• Use NZB360/Web-UI to manually search or *arr for monitored stuff
• *arr pushes download to rutorrent/rtorrent via XML-RPC
• seedbox downloads and makes a hardlink to a folder (/seedbox/files/resilio/). Since I tag my torrents, rtorrent also creates a subfolder in resilio called sonarr, radarr or lidarr.
• resilio notices that and syncs with my home node.
• Remote path mapping in the *arrs take care of the automatic import (there will be a few errors in the log until it’s fully synced)
• (For me it’s an issue, maybe I set it up wrong) *arrs will import it by copying the file from /data/seedbox/<*arr> to the respective media library like movie, tv, anime, music, etc.
• Seedbox continues to do it’s thing.

Once in a while I have to manually clean the sync folder since I have a semi-broken setup.