Semi-private just refers to how easy it is to join them. E.g. rutracker is considered a semi-private tracker, because it requires an account, but always allows registrations and does not enforce any ratio.
In that sense I was wrong in calling TL a semi-private tracker, because TL does require maintaining a ratio. But given it is possible to simply join via their seedbox offerings, it is not as private as some other trackers, which require proofs of good behaviour on other trackers and/or an application process.
Edit:
Public: no registration required
Semi-private: registration required, but always possible; lax ratio rules
Private: registration required, mostly through invites/applications; anti-leech ratio rules
It depends on what trackers you’re on and how much storage you have, and how risk averse you are.
First of all, binding your torrent client to the VPN interface should prevent all leakage.
Additional precautions like running your torrent client behind a container like gluetun should make it pretty much impossible to leak your IP to adversaries. Or if you have a plain Linux server, running the torrent client in it’s own network namespace also achieves the same result.
The other big reason to get a seedbox is to be able to maintain your ratio. This depends on your tracker.
E.g. I have enough storage for a large enough seeding size and enought torrents to get sufficient bonus points. Combined with a bit of upload here and there, I get enough upload/buffer to snatch what I want.
On many trackers, large enough torrents are often freeleech, so they don’t count towards the download stat anyway.
tl;dr
If you bound your torrent client to the VPN, I’d seed with your NAS unless you don’t get enough upload to maintain your ratio on your specific private trackers. Storage is way cheaper on your NAS.