I used to only use Audacity, it actually works pretty well for surround sound, but it's kind of clunky & you'll need to utilize more 3rd-party plug-ins than you would if you just used Adobe Audition, which has a lot of built-in effects, processes, utilities, etc. that are engineered for 5.1 audio. I mix A LOT of songs in surround, do it everyday & I mainly use Audition. If you're interested, there's many YouTube videos, forums, & also a subreddit for tutorials & such.
I do still use Audacity to create mixes with more than 6 channels (5.1.4, 7.1, 7.1.4, etc) but the bulk of the mixing is still done with Adobe. One day I might attempt to switch to Reaper & see what all the fuss is about. If you decide on Audition & have trouble figuring something out, I might be able to help.