First & foremost, this is a problem best solved by HubSpot. Please upvote @STaruc feature request so HubSpot will fix this for us: https://community.hubspot.com/t5/HubSpot-Ideas/Google-Enhanced-Conversions-through-HubSpot/idi-p/596191
HubSpot support just told me that adding a delay to the ‘thank you’ page loading is probably not the best solution. It can take 30 seconds or more for the Personalization data to be made available.
So, I think the only viable solution for now, as suggested by @STaruc , is this: https://community.hubspot.com/t5/APIs-Integrations/Include-form-fields-as-redirect-URL-parameters-on-an-embedded/td-p/235041