as you are trying to sharpen specific skills i would suggest you to focus only on Advanced cases. You really need to get out of your confort zone to improve your skills: you will benefit the most from cases you won't be able to solve at the first instance.
Based on your need, I would suggest you to start with the following cases from PrepLounge library:
- REA reinsurance
- Universal airlines
Please le me know if you need any further help on preparing on strategic/ quantitative cases. If you have already done all the cases here/ you find them to easy I could help you with some cases from my porsonal collection