Normally you would want to level in whatever you record so the raw peaks won't go into the red (no clipping) but have some more or less comfortable headroom. This should be done with some sort of mixer that is somewhere in the space from after the microphone to the audio interface, monitoring the peaks as they would get digitally recorded. Further steps are subject to mixing the sounds. If your headroom needs to be very large, then gainers can ofc be used to bring the audio into comparable level of other tracks, but you will need to take care of the peaks then somewhere in the chain (for example with proper use of saturation plugs or limiters) before stuff hits the master out, else you will have scratchy sounds for the clipping peaks.
If I understood this right your problem is that the slap bass is much louder in the slaps than in the mellower picking. That is a normal trait of especially clean slap bass. And a well known challenge in mixing. You can try googling it, it is really one common problem, I remember at least 2 mixing faqs I just had on my screen where the slap bass was choosen as one of the more extreme examples when it came to bringing tracks to even level for the mix. Solutions mostly involve use of compressors and some equalisation, but needs to be very fine tuned to level out the bass so both slapped and picked notes can be heard well and with impact in the mix.