I received a response from Adobe for my use case. Transaction ID doesn't increment visitor metrics, but it is associated with the visitor id. Since it is associated I was able to get my intended functionality for my eVar by creating a segment.

For reference, as a test I am trying to tie a single eVar uploaded with Data Sources via the Data Sources API to an existing visitor in Analytics (using Adobe Launch). I am using Experience Cloud Visitor ID as transactionID, event "Data Source Upload", and an eVar where I am sending a string "bot". ...