Hi @Stevie_Simon ,
sorry to hear that you run into trouble. S1 is usually a very good vst3 host, so HALion 7 should work. Before sending you to Presonus or Steinberg support, we can try a few steps for due diligence troubleshooting.
You are right in your expectation that the Steinberg Download Assistant should have handled everything correctly. To be “closer” to the system, I would like to ask you to run the downloaded installer directly.
You can access the download folder with the following steps on the screenshot below:
If there are any errors during the installation or if S1 can still not find the instrument, please let me know.