Big thank you to OP for sharing their solution!

Just to add my findings when I used it these days on the iOS Safari browser …

• This line in the html code does not work properly for me as it returns an object:

google.script.run.withSuccessHandler(google.script.host.close).passResult(result);

• I got the expected result after serializing the object as a JSON string:

google.script.run.withSuccessHandler(google.script.host.close).passResult(JSON.stringify(result));

• Accordingly, the JSON string has to be parsed in the GAS code as well …
  
  in function passResult(result):

const parsedResult = JSON.parse(result);