Your question points a typical challenge that candidates face in preparing for case interviews, and developping this ability is a key success factor, if not the most important.
This will of course require a lot of practice and will never be resolved by reading a few tips, but let's start with some basic advices that most candidate forget about / don't know about.
1. (almost) Never use a standard framework taken from the books. Strategy consulting is about helping client facing unique issue with a tailored answer. Forcing your approach in standard framework is unlikely to be relevant
2. Project yourself in your clients shoes and show emphaty for its problematic. By doing so you this will be much easier for you to understand what are the key topics to cover in order to formulate a recommandation and make sure you don't forget anything. I always ask myself "what would I do if this was my business and my own problem ? What do I need to know / understand to make a decision ?"
3. Make sure every topic you want to cover is relavant for the final recommandation. A simple check is to ask yourslef "If I spend time on this specific topic, and get some answers to my questions (ex. market sizing, competition, etc.), will this bring usefull element for the final recommandation given my clients problematics" ? If the answer is no, then you should drop this sub issue.
4. Practice a lot, with any daily case you can set up on your own. The above tips are taken from my own experience doing my best to build MECE structures, but keep in mind that it takes a lot of practice to achieve a satisfying performance here. I have developped an exercice called structuring drills, that focus your effort on learning to build structure. This is ususally very appreciated by candidates. Don't hesitat to ask more by mp if you need help here.
Hope this will help