I have tried several different mirrors and my favorite for years now is the Bike Peddler Take-a-look. Stable, easy to mount and with a good field of view. Mounting it to the helmet may require zip ties, depending on design, but you can also mount it onto your glasses.
I have gotten so used to it that I catch myself looking to the upper left, even when I am not riding my bike and not wearing my helmet.
I've tried the little mirror and didn't really like it. You have to focus too much on the mirror to see. I've also tried the mirror attached to the bars and its a little easier but my arm blocks some of the view. I currently have the rear camera option tied to a small 12v battery. I replaced the battery with a 18v 1.5 ah battery that has an on/off switch. That lasts about 4 hours. Cost was $30 on Amazon.
This unit handles 6v-36v I believe. So a 9v battery would work. I tried that too.
Would be nice to get a screen without the backup lines. Like this one...
This morning I took a picture of the camera in low light. I don't see the horizontal lines looking at the screen so I'm not sure why its in the picture.
I added some small lights near it to see if that helps. It probably doesn't help during the day but more at night. It's a cloudy day so sometimes its dark and other times its bright. I can also try moving the camera elsewhere on the bike. Maybe the frame and rear lights are blocking some of the light.