Is there any possibility to limit the bandwidth of an SFTP user? My server has an upload of ~500kb/s and I don't want to 'spend' it all on one user
If you are using WHM (Web Host Manager), you can simply go to "Limit Bandwidth Usage" under the "Account Functions" section. Choose an account and then enter a limit.
If not, you can use a utility called trickle - https://github.com/mariusae/trickle
For openssh, see man sftp
-l limit Limits the used bandwidth, specified in Kbit/s.