When the 3rd party cookies are blocked, the Visitor ID is not being shared between different domains.
I have implemented a custom code to accomplish the Visitor.appendVisitorIDsTo(URL) solution:
https://experienceleague.adobe.com/docs/id-service/using/id-service-api/methods/appendvisitorid.html...
Don't you think that it would be useful to create an extension to accomplish this in an easy way? Something like Google Tag Manager does for this scenario that it's a simple checkbox specifying that you need "cross-domain" tracking.
I think that this is not something difficult and it would help a lot of "non-technical" people.
Regards.