Strategy for Assumption questions

I hear you. CR is a bit tough for non natives because it takes time to analyze the argument. I'm not going to overload you with suggestions. Here are the 2 things that worked for me:

1.) "conclusion is the king". The first step is to identify the conclusion and the second is to attack it (and not the premise). When you practice tons of questions, your brain will get accustomed to remove the chaff from the grain (premise from the conclusion). You will take a lot of time initially, to separate out the conclusion & the premise, but its worth practicing. After a few hundred questions, you will not even stop to think what is what and will jump on the options straight away.

2.) Pre-think. It improves your analysis and forces you to create a structure. Lastly, have the courage to use the same process on the exam. Many of us practice one thing and follow a different routine on the exam. Avoid that.
The best part: pre-thinking works for a lot of questions. Strengthening, Weakening, Assumptions, Flaw, Evaluate and even for main idea of RC passages. You may use this to gain some understanding of the concept: https://e-gmat.com/blogs/?p=1750

Having studied for so long, you might have exhausted all the official questions. Don't worry too much. Try to solve them with a new approach and you will find improvement in your score.

good luck!