I think this is more valuable for the plugin to support Enterprise users.
I tried your suggestion for the endpoint. I get a fatal error because $obj->endpoint is protected.
Error: Cannot access protected property WPSparkPost\SparkPostHTTPMailer::$endpoint
We thought that there is a field to do a fork and a pull request where you could review the code and validate it.
Do you think your team could welcome this option as an opportunity of improvement?
Thank you