When it comes to mantras, it's the intention behind it that makes it work rather than the words themselves. If you have enough intention upon waking up to stay still, you will. The mantra is just there to maintain your intention.

You shouldn't need the first mantra; you already naturally wake up after dreams and you're already setting the same intention to notice your awakenings with your second mantra.

If the mantra is short, it will be easier to repeat. For example, if I wanted to stay still upon waking up, my mantra would be, "Stay still when waking up".

Good luck with your technique!