Is there no

before update

after update

options when running the SDFInstallation script? What if I need to gather data BEFORE the update .. .then using that data AFTER the update is complete?

You should be able to control when the script executes based on when you set the script to run in the deploy.xml. Then you can compare the version that comes through. If there is no current version, then it’s a fresh install

Really, where can I read about the sequencing in the deploy.xml?

You should be able to put your script before the files and objects in the deploy

<deploy>
      <run>
          <script>
               <path>~/Objects/customscript_install_script.xml</path>
               <deployment>customdeploy1</deployment>
          </script>
     </run>
<files>
        <path>~/FileCabinet/SuiteApps/com.example/*</path>
<files/>
    <objects>
        <path>~/Objects/*</path>
    </objects>
</deploy>

I'll give it a go. Thank @Matt Bernstein!