Am I sinful if I intentionally do kufr in my mind but don’t act upon it out of fear of Allah ?
Wa’alaykum as Salam wa rahmatullahi wa barakatuhu,
Once you get a thought, you should not willingly entertain it. As soon as the though comes, you should recite Ta’awudh.
If you do this, then as long as the thought it just in your mind, and you do not physically practice on it, you will not be sinful.