I don't own a mini, only small and med, but I think you're right about taking longer to slouch. However, you can accelerate it by using it daily, and carrying what you need without worrying that it'll lose shape. Sturdy is good when it comes to leather integrity, and wear and tear. Dont worry and start using it. By nature it's a slouchy bag so give it some time and it will get more mushy.
If you truly want slouchy right away, I'm sure there are preloved ones that you can get deals from that will come already slouched.