Multi-party decryption, no central decrypter
I just wanted to give you a hint, but since I cannot write comments under your question, I write it here:
I think you are searching for a scheme that whenever the secret is reconstructed, the participant's keys dont reveal. We have such schemes, please see this Lecture Note Chaper 6.

Lecture Note information: Lecture Notes, Cryptographic Protocols, Version 1.21
September 13, 2016, Berry Schoenmakers
The name of the protocol is: Threshold Elgamal Cryptosystem  But the whole chapter that I mentioned is more informative than just this protocol.