Can I still claim the novelty of my PhD contribution in later publications? I understand that it is common to publish a number of conference papers while working on PhD and, then, publish conference/journal articles from your thesis afterwards.

However, when the main contribution has already been introduced in the thesis, can I still claim that the contribution is novel?
I think it depends on the venue (if any) in which your dissertation has been disseminated.

However, it is likely you can appropriately claim the subsequent manuscript aimed for a peer reviewed journal is indeed the first report of your claims in the peer reviewed scholarly literature.

However, do disclose any prior dissemination to avoid accuses of self-plagiarism.