Maybe Linux just isn’t for you, and that’s okay. Go use Windows or Mac and enjoy your “just works” setup and lack of involuntary learning.

All the CDLC for RS2014 work by pretending to be the DLC for “Smashing Pumpkins - Cherub Rock”, because it was given away for free as a preorder. If you get any of the other delisted DLC, you can use Rocksmith Custom Song Toolkit to change the ID to one you own (free or paid, as long as you have a steam license for it) and it should work.