Good evening Michael and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
I believe that in your situation, you will probably be better served by upgrading to the agency license than by upgrading to the multisite license.
The reason for this is because you can indeed split these 10 license keys anywhere you want to.
By contrast, the multisite license is really meant to stay on one multi-site network.
As a starting point, I would recommend reading the following: https://theeventscalendar.com/knowledgebase/multisite-license/
Let me know if that helps.
Have a great day!
Geoff B.