I strongly suggest that you use something like Network Namespaces through Vopono[0] or Gluetun[1] if you use a commercial VPN for "privacy" or "security" aka torrenting and shitposting. Relying on these clients is always a gamble and if your software (Browser, Torrentclient, etc.) cannot know you public IP only the internal IP of the VPN you are also safe against some exploits and misconfigurations a desktop client won't protect you against.

[0] https://github.com/jamesmcm/vopono [1] https://github.com/qdm12/gluetun

Wouldn't blocking IPv6 and using a kill-switch prevent leaking?

I strongly suggest you disable ipv6, as nothing will break by disabling it but many things break with it enabled.