My team is in the same boat using the HubSpot integration with WordPress. We use HubSpot forms on a WordPress site with WordPress Thank You pages, and our Google Rep wants us to add form field data on the Thank You pages in addition to creating the GTM conversion tag (we used a non-GTM conversion tag prior). Not sure what our options are - open to moving our Thank You pages to HubSpot if that helps things along - but seeing some of the other replies here it looks like that may not be viable at this time.