Hi @PD10 ,
You have few options here -
1. Roll up Report Suite - Create a roll up report suite from all of the report suites in questions given they are under one same organization. Caveat will be loss of de-duplication.
2. Multi-Suite Tagging - Send all of your data in one Global report suite and one let's call it local report suite. You will have all your data in one plus other one for localized analysis. Caveat here being increase in server call volume.
3. Only one Global Report Suite - You can also create just one Global report suite. In that, as you mentioned, create an eVar to identify the site or locate so you can create virtual report suite or segmented data when you need. This would be cost-effective solution but you will miss out on some of the report suite benefits in virtual one.
We are doing in the 3rd way in my organization.
Link - https://experienceleague.adobe.com/en/docs/analytics/components/virtual-report-suites/vrs-considerat...