here is a diagram of the brz/frs exhaust system for your reference:
You are correct in saying that replacing the front pipe only is easier. If you were to do an over pipe or and OP/FP combo, you would have to raise the engine. But if you say you are good with a wrench, it is super easy to do.
Simply having a resonator wont reduce rasp. A resonator will reduce or remove droning. A resonator is designed to delete the specific sound waves that cause drone. The only way to really reduce rasp is to keep the cats in the exhaust system.
If you really don't want to replace the over pipe, Nameless (and just to mention I am not affiliated with them at all they just make some really top quality stuff!) also makes just a down pipe that you can get with a resonator and cat.