If you do rune stick with rune, Sanskrit AUMs stick with those, necronomicon, stick with that, etc. Keep it consistent in that regard amongst all chakras for a session or for opening, otherwise use whatever you want. Sanskrit is most powerful. But maybe too powerful if you’re just starting off. It’s what you can handle and feel comfortable with.
If a mantra is a struggle for you, even if it is powerful, you’re better off doing something else cause you’re not getting the full power of that mantra if you can’t vibrate it correctly. But also it’s worth practicing cause they’re not all that hard if you practice. Kaun is just a hard K and then AAA-UUU-NNNN. Not too hard.