Relate an Email Thread to a Salesforce object

Question

We are using Einstein Activity Capture in order to log in emails that we have with Specific Contacts.  Our contacts are most of the time involved in Many opportunities so a specific contact can be linked to a dozen of opportunities

By using Einstein we are able to link email (Inbound and outbound) to a specific contact but since they are related to multiple deals (opportunities) then everything is mixed and there is no way to keep up.. 

It is why we would like to link a specific thread to an opportunity that would record automatically in the object activity all the mails that are in that thread .. inbound and outbound 

We are using the Gmail Extension provided by Salesforce...

​​​​​​​Please helppp ;) 

Answer 1

EDIT: Einstein Activity Capture has limited email capture for opportunities.

If Einstein Activity Capture adds emails and events to a Contact or Person Account that’s associated with an Opportunity’s Contact Role, those emails and events are also added to the Opportunity. Einstein Activity Capture only adds emails and events that occurred in an Opportunity's date range to the Opportunity.

Apart from this, you cannot create custom links or relationships because EAC activities are stored externally. See this Knowledgebase article:

please note that the emails streamed/captured in Salesforce using EAC are not created as records and are not saved in the database of Salesforce as 'Email Messages', unlike the emails added manually

Captured activities are also stored externally:

Because activities added to Salesforce with Einstein Activity Capture aren’t stored in Salesforce, they don’t show up in standard Salesforce reports