I read this as that the action is too low for you to play slap, and that you want to raise the fretboard as part of the fix.
Does that mean you're going to raise the bridge so you can get your plucking fingers under the strings, and then raise the fretboard correspondingly so the action there is still low?
Won't that mean the strings bounce and buzz on the frets all the time as they are now vibrating vertically?
Sorry if I'm being thick here, I don't play slap and I'm just trying to understand the problem. I'm also not a luthier so I can't help practically either, sorry. ☹️