If you play shelby song in carbuncles, you would play it like a falkow file. I have seen this once till now with the new ex and jade. While its fun I think its suboptimal and refess has much better tools to build and maintain their field.
None of the lvl up cards appear to be directly useful for your buncle file, yet if you plan to ever create another file on that account, shelby may be a good long term investment. Else just pick a 5* and recycle it when you see the oportunity to buff up your buncle file which a good sale later.