It is possible to make the fields mandatory on a Transition. This is native functionality, in Jira Cloud. You can use the Validators on the workflow Transition.
In the Field Configuration, when you make a field mandatory, it will add a red * on the field, in the create and edit screens.
But as far as I know, it won't mark a mandatory field set on a Transition. You just get an error stating that the field is mandatory when trying to transition the issue.
You can do this using the Power Scripts add-on, which can also do countless other things you will likely want to do with Jira. The only thing it won't do is add the * but you can have it give a custom error message.
Here is a video tutorial walking you through How to require attachments.
I hope this helps!
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