First of all, I think it is not a good approach to allow the user to reply to multiple comments at the same time before hitting save. And each of the replies can be a separate components, that way each one will handle it's own state.
Now to answer your try from above, you cannot have $replyToComment variable, and then append just the comment id, you will need to store the replies in an array and then loop through it in order to store the replies:
// in component
public $repliesToComments = [];
// and to save them in a function you'll add
foreach($this->repliesToComments as $commentId => $message)
{
// you got the id and the message here.
}
// in view
<div>
@foreach ($comments as $comment)
<h5> {{$comment->text}}</h5>
<div class="input-group mb-3">
<textarea rows="1" wire:model="repliesToComments[{{$comment->id}}]"></textarea>
</div>
@endforeach
</div>
But make sure you consider my initial comments above, as that makes more sense.