Audacity could be FreshJamz (Jade's music player). MuseScore could also be that but I was thinking disguising MuseScore as OrgMaker or a parody of OrgMaker (DAW used by homestuck musicians).

Trash could be a graveyard (like ~ATH) or simply named "Deltritus"

Deltarune is Undertale: Beyond Canon (THIS IS A JOKE IM JOKING!!!)

As for the background, I'd suggest think of the media you've hyperfixated upon most recently or fondly regarded the most (that isn't homestuck). John has ghostbusters, Jane has Problem Sleuth, I think Roxy's is just a swet wozard.

Alternatively, a piece of art you created (Dave and Dirk both use And It Don't Stop, an earlier comic by Hussie, as their backgrounds iirc)

(...Steam could be Complete Bullshit, or MSPaintAdventures itself since the comics are meant to be "games")