Maybe you could also consider to extend an existing good PQC protocol (for example https://signal.org/docs/specifications/pqxdh/) by adding a shared secret. The shared secret could flow into the key derivation functions which are used to derive the symmetric encryption keys. This way you would have quite strong guarantees (forward secrecy) as long as nobody can break the PQC algorithm and still some guarantee of confidentiality when somebody breaks asymmetric primitives. In the protocol you outlined now, there's no forward secrecy. Meaning that once a key is compromised, all past and future messages can be decrypted.