A suggestion and a comment for you:
1) Try striking THROUGH the A string and having your thumb come to rest on the D string. Think of it as "rest-stroke" technique for your thumb. This might change the angle of your thumb a bit and help you to miss the E string. It also helps to reduce the amount of motion you need to hit the string a bit as well as setting you up for a "reverse thumb" technique later on down the line.
Also, it should all come from your wrist for now - think of grasping a doorknob with your right hand and turning it back and forth quickly several times. Your forearm shouldn't be moving other than the fact that it's rotating like a rolling pin. This will help your consistency once your aim is improved.
Please note that this is one of possibly HUNDREDS of perfectly valid techniques for playing "slap style", and in this style of bass playing, the end justifies the means - meaning however you're able to get the sound you want is just fine.
2) Whenever you strike a string, no matter how perfect your technique is, the other strings are GOING to ring. The vibration is too much for the strings to lay silent. So you'll want to work on muting techniques for the left hand as well. Fingers that aren't busy holding down notes can be used to keep other strings from ringing. Muting techniques are too individual to really spend a lot of time typing about it, so you'll have to experiment to find what works best for you.
Keep at it - this isn't instant, and some of these techniques take longer to master than others for different people. It took me ten years to get proficient at the "reverse thumb" concept. Here's hoping you get where you want to go in a shorter period of time.
