ChaCha20-Poly1305: Can my salt/pass for a KDF also be the nonce?
If you have a PSK then you don't need a password. You can just derive new session keys from the PSK, e.g. using a salt within HKDF. It is also possible to derive a key and IV if you're already using a salt. You could also use a counter starting at zero as nonce / IV instead because the resulting key is considered "fresh".

Generally, you simply need a unique key nonce combination for any encryption operation for stream ciphers. If you always apply a random salt to derive both than that requirement is fulfilled, because the key is always fresh. Reusing the salt is however immediately fatal to the security, so the salt should consist of say, 12 bytes / 96 bits or more generated by a secure random number generator.



Using a random salt together with a PBKDF is also possible, but as indicated, I'm not sure if you need a password and therefore a password stretching function such as a password hash / PBKDF.