@Tilbert Your ISP might throttle your speeds based on your recent behavior if that is allowed in your country and stated in your agreement.
What I advise you to do is, look for a torrent with high popularity in a public tracker, for instance a TV show episode that was just posted. Let it sit in your client for a few hours and see how the upload speed goes.
If it caps around 600 kbps, it could be either the ISP or the VPN. You can try disabling the VPN and checking if it has any effect. In case it stays the same, you can try temporarily switching to a different network, just to confirm.
If you cannot connect to a different wifi, you can share your phone's 4G/5G to your PC using a USB cable, and see if it has any effect on upload speed.