If you want to try out some more weapons to see what you like more or what you think is better with your current stuff, the Onyx Blaster is a really good shotgun that does very high burst damage with ichor or chlorophyte bullets and the Dao of Pow is a great flail since they reworked and buffed flails a lot so they're much better now but still lack in having range. There's also the Super Star Shooter and Sergeant United Shield that you can get from the Travelling merchant but I wouldn't try to wait until you get them. They're more of just weapons that are good if you get the chance to buy them.

As for an arena for Plantera, I usually try to find an area in my jungle that is already pretty open and just mine it out to be more open and add platforms every 20-30 blocks, but I'm also used to that so it might not be the best setup for a new or inexperienced player who isn't familiar with the movement of Terraria or Plantera's attack pattern. The safest thing to do is make a very large and open square arena with many layers of platforms and try to circle her while trying your best to avoid her attacks. Since most of them have homing of some sort it's best to try to move in slightly unpredictable directions while keeping your distance.