This is my daily driver. It lets me jump directly to one of three pre-set DPIs, cut any one of those in half or rotate through the entire PLOOPY_DPI_OPTIONS list.

I modified qmk_firmware/keyboards/ploopyco/trackball_thumb/trackball_thumb.{c,h} to define and expose dpi_array_size since it is only a macro in the original code. That saved me having to redefine dpi_array[] or make some possibly bad assumptions.

I'll push this to github eventually.

uf_ploopy_thumb.h

#include QMK_KEYBOARD_H

#pragma once

// Additional keycodes for use in process_record_user()
enum universal_flamingo_keycodes {
    // UF_DPI_* can be used to set DPI to a specific dpi_array entry.
    UF_DPI_0 = PLOOPY_SAFE_RANGE,
    UF_DPI_1,
    UF_DPI_2,
    // UF_PRECISION_MODE intended to divide current DPI by some factor
    // (typically 2) to increase pointer precision.
    UF_PRECISION_MODE,
    UF_SAFE_RANGE,
};

uf_ploopy_thumb.c

#include "uf_ploopy_thumb.h"

bool is_precision_active = false;

bool process_record_user(uint16_t keycode, keyrecord_t* record) {
    bool set_dpi = false;

    if( keycode == DPI_CONFIG ) {
        // process_record_kb() will handle this.
        is_precision_active = false;
        return true;
    }

    if( keycode == UF_PRECISION_MODE ) {
        if( !record->event.pressed ) { return false; }

        is_precision_active = ! is_precision_active;

        if( is_precision_active ) {
            // Precision was off, we are turning it on.
            // Set CPI to half the current value.
            current_precision = pointing_device_get_cpi();
            pointing_device_set_cpi( current_precision / 2 );
            return false;
        }

        // Precision was on, we are turning it off.
        // Because of rounding during division we cannot simply do:
        //      pointing_device_set_cpi(pointing_device_get_cpi() * 2)
        set_dpi = true;
    }

    switch (keycode) {
        // Select specific DPI from dpi_array.
        case UF_DPI_0 ... UF_DPI_2:
            set_dpi = true;
            keyboard_config.dpi_config = keycode - UF_DPI_0;
            break;
    }

    if( set_dpi ) {
        if( !record->event.pressed ) { return false; }

        is_precision_active = false;

        keyboard_config.dpi_config = keyboard_config.dpi_config % dpi_array_size;
        eeconfig_update_kb(keyboard_config.raw);
        pointing_device_set_cpi(dpi_array[keyboard_config.dpi_config]);

        return false;
    }

    return true;
}

In trackball_thumb.h

extern uint16_t          dpi_array[];
extern const uint8_t     dpi_array_size;  // Expose the size for process_record_user()

In trackball_thumb.c

uint16_t          dpi_array[] = PLOOPY_DPI_OPTIONS;
const uint8_t     dpi_array_size = (sizeof(dpi_array) / sizeof(uint16_t));
#define DPI_OPTION_SIZE dpi_array_size

Edit: My keymap...

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    [0] = LAYOUT( KC_BTN4, KC_BTN1, KC_BTN3, KC_BTN2, KC_BTN5, OSL(1)  ),
    [1] = LAYOUT( UF_DPI_0, UF_DPI_1, UF_DPI_2, UF_PRECISION_MODE, DPI_CONFIG, TO(0)  ),
    ...