I have a little block plane, a very cheap one, that I have used to plane down the outside top corner of the plastic trim almost to the glass, all the way around. Then the rim breaks at that corner when I pry it up. The inside half is relatively easy to remove after that. Sawing through the trim near each corner as the OP did is a very good idea too. And, I found that forcing myself to be patient was the hardest and most effective technique.
Alex009 has an excellent writeup here, which I wish I had read before I did my two rim removals. If I had, I think I would have left the corners on too.