In a suitelet, I have a SELECT that needs to control the options available on another select in the same suitelet… is there a way to contain this logic inside the suitelet itself or do I need to create a clientscript that runs on top of the suitelet?

That would indeed be a client script

You can follow this article, hope this helps: https://leacc.com.ph/2019/11/06/suitescript-2-0-sample-code-for-filtering-a-dropdown-field/

I believe it will also work for external suitelet ? @Lea CC @Budy Sutjijati

Yes