I want to enter developer more, How do you see Memory usage in Android O?, but am wondering if there are any dangers in doing so. Obviously I could change things and mess up, but will I open security holes to other apps by being in developer mode? Or are there other issues I need to consider?
No, it's completely safe to enable developer options.
Doing so only lets you access these options from your phone settings. Some of them may be dangerous under some circumstances, but they have to be explicitly enabled by you, as they are not activated automatically.
Developer mode is nothing near to the "REAL DANGER" on android.
It's safe for most part unless you click or change something you should not.
If you are just a casual android user go ahead.
If you want really very very technical insight (and probably thilling work) on android, you can try rooting your phone.
I neither recommend nor force to any harm done to your smartphone especially by rooting it.
Remember, Rooting your phone voids it's warranty.