HT with Rebels is easy with Falcon.
If you have a finished TIE bomber, I've seen a Thrawn Imperial fleet on my shard somehow manage to be competitive against Negotiator and Malevolence, so it's doable.
However, it sounds like your Galactic Republic is coming along--Anakin and Ahsoka at r7. Getting Negotiator seems like your best path. Which sounds like it would mean switching guilds.
If you don't want to switch guilds, you can try getting Falcon (Rebel Y-Wing would be necessary if you want to fight Negotiator though) or Chimaera. Otherwise be content to not be competitive in arena if you are staying in a low power guild.