Every single official guidance on GDPR that I have seen, such as https://ico.org.uk/for-organisations/guide-to-dp/guide-to-th..., states that I would have conflict of interest serving as DPO because "Basically this means the DPO cannot hold a position within your organization that leads him or her to determine the purposes and the means of the processing of personal data."

The same document specifically points out that as I head marketing, I cannot also the the DPO.