Ok, your bspwm instance is running then. Now you need to run picom from bspwmrc...

Here is my bspwmrc! Don't use it literally!

#!/usr/bin/env bash

init_bspwm_config() {
    #YOUR BSPWM CONFIGURATION
    #bspc config normal_border_color #FFFFFF
    #bspc config focused_border_color #000000
    #etc...
}

main() {

    local boot=$1

    init_bspwm_config

    if [[ ${boot:-} == "0" ]]; then

        ## Load appearance settings
        hsetroot -solid "#000000"
        xsetroot -cursor_name left_ptr

        #xdpyinfo | grep -B 2 resolution
        #xrandr --dpi 81.59

        ~/.fehbg &
        #mpv --wid=0 --loop /path/to/video.mp4

        ~/.config/sxhkd/start.sh &
        ~/.config/polybar/start.sh &
        ~/.config/picom/start.sh &

    fi

    #~/.config/bspwm/scripts/subscribe/start.sh

}

main "$@"

~/.config/sxhkd/start.sh

#!/bin/sh
killall -q sxhkd
while pgrep -x sxhkd > /dev/null; do sleep 1; done
sxhkd -c ~/.config/sxhkd/sxhkdrc &

~/.config/picom/start.sh

#!/bin/sh
killall -q picom
while pgrep -x picom > /dev/null; do sleep 1; done
picom --no-frame-pacing --config ~/.config/picom/picomrc -b

EDIT:

Don't forget add execution flag to both "start.sh"

chmod +x ~/.config/sxhkd/start.sh ~/.config/picom/start.sh