You probably just want to restrict the topic custom field input to displaying in the composer for the first post, and not in the composer for replies. So add a conditional wrapper around the input
<TopicCustomFieldInput
@fieldValue={{this.fieldValue}}
@onChangeField={{this.onChangeField}}
/>
See if you can figure out what attribute you need to check for when restricting the input. Start by seeing what arguments are passed to the composer-fields plugin outlet where this input is added.