Buy a metronome and pratice slooowwlyyy. i you learn poor technique at correct speed, it's very hard to iron out. Learn it slowly and you can nip the imperfections in the bud, learn where your hands are supposed to be, and importantly with slap, sort out left hand muting to stop accidently hit string ringing out. When you bring it up to speed, and should sound great. (at least that's the theory

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This goes for other techniques as well. when I was first advised to practice at snail's pace, I tried playing some of my existing chops slowly. I was horrified at how untify my technique was. I sorted out the mistakes and dosdy technque, and the played them up to speed, and was amazed how much better they sounded.
PS, the thumb is a fine slap bass ( i play the same thumb 5 BO). bell brass frets, set back P/ups ans a ton of string/body clearance. The 2 band eq is quite useful too, for getting a scooped slap tone.