You should stop thinking about it and do it (soon). I suspect it will be tough if you master all the songs with guitar and post some high numbers to go back and try to grind through the same songs again on bass. Unless you are really motiviated to learn bass later on, I suspect you might loose interest, I know I would. I am using the Bass DLC to really work on my long neglected fingerstyle techinque. I am using the the newness of the bass lines and the fun of the game to push me through the grind. If I was just learning a setlist for a gig I would have resorted to a pick already (which I already know how to do - more or less).
Once I get the fingerstyle down, I can't wait to learn slap.
Heck, you're already a drummer, time to complete the set.

Besides having a Bass around the house is great for doing your own recordings, backing tracks etc.
Have I taunted you enough for one day?