You can do TLS with shared keys. The rest of this comment describes the situation in TLS 1.3 since earlier versions are not used in QUIC and future versions don't exist yet.
If you want Forward Secrecy you still need to do the DH steps so that you'll get unique session keys, but you don't need a certificate since your mutual knowledge of the shared key is sufficient.
If you don't need Forward Secrecy (pro tip: You actually do, but I can't stop you) then you can skip DH and just go straight to the main course.
If you want Forward Secrecy you still need to do the DH steps so that you'll get unique session keys, but you don't need a certificate since your mutual knowledge of the shared key is sufficient.
If you don't need Forward Secrecy (pro tip: You actually do, but I can't stop you) then you can skip DH and just go straight to the main course.