Is it permissible to trim new hair growing from the eyebrow – that was not previously present – after having stopped removing eyebrow hair that was thick and bothersome?

You have done well by abandoning plucking (al-Nams) once its prohibition became clear. The default ruling is the prohibition of taking from the eyebrow hairs. However, if the new growth of hair makes the shape of the eyebrows deviate from their natural form, and their deviation is apparent, noticeable, and unsightly, then it is permissible to remove from them only the amount that makes them not unsightly.