Tap Dance and Mouse Key. You can define the hold to be CMD or layer.

mouse-tap-sample.c

#define ACTION_TAP_DANCE_MOUSE_LAYER(kc1, layer) \
  { .fn = {mousekey_tap, mousekey_finished, mousekey_reset}, .user_data = (void *)&((qk_tap_dance_pair_t){kc1, layer}),  }

// Tap Dance declarations
enum {
    TD_MB_CMD,
    TD_MB_L1,
    TD_RMB_L2
};

enum {
    SINGLE_TAP = 1,
    SINGLE_HOLD,
    DOUBLE_TAP,
    DOUBLE_HOLD,
    DOUBLE_SINGLE_TAP, // Send two single taps
    TRIPLE_TAP,
    TRIPLE_HOLD,
    IGNORE_TAP,
    SHIFTED_ON_INITAL
};

static tap mtap_state = {
  .is_press_action = true,
  .state = 0,
  .layer = -1
};

void mousekey_tap (qk_tap_dance_state_t *state, void *user_data);
void mousekey_finished (qk_tap_dance_state_t *state, void *user_data);
void mousekey_reset (qk_tap_dance_state_t *state, void *user_data);

// Tap Dance definitions
qk_tap_dance_action_t tap_dance_actions[] = {
  [TD_MB_CMD] = { .fn = {mousekey_tap, mousekey_finished, mousekey_reset}, .user_data = NULL },
  [TD_MB_L1] = ACTION_TAP_DANCE_MOUSE_LAYER(KC_MS_BTN1, 1),
  [TD_RMB_L2] = ACTION_TAP_DANCE_MOUSE_LAYER(KC_MS_BTN2, 2)
}

void mousekey_tap (qk_tap_dance_state_t *state, void *user_data) {
  if (state->count == 1) {
    mtap_state.layer = -1;
  }

  if (user_data) {
    qk_tap_dance_pair_t *pair = (qk_tap_dance_pair_t *)user_data;
    mtap_state.layer = pair->kc2;
  }
}

void mousekey_finished (qk_tap_dance_state_t *state, void *user_data) {
  uint16_t mouse_key = KC_MS_BTN1;
  int layer = -1;

  if (user_data) {
    qk_tap_dance_pair_t *pair = (qk_tap_dance_pair_t *)user_data;
    mouse_key = pair->kc1;
    layer = pair->kc2;
  }

  uint8_t st = SINGLE_TAP;

  if (state->count == 1) {
      if (!state->pressed) {
        st = SINGLE_TAP;
      } else {
        st = SINGLE_HOLD;
      }
  } else if (state->count == 2) {
      if (state->pressed) st = DOUBLE_HOLD; else st = DOUBLE_TAP;
  } if (state->count == 3) {
      if (state->interrupted || !state->pressed) st = TRIPLE_TAP;
      else st = DOUBLE_SINGLE_TAP;
  }

  mtap_state.state = st;
  switch (mtap_state.state) {
    case SINGLE_HOLD: if (layer > -1) layer_on(layer); else register_code(KC_LGUI); break;
    case SINGLE_TAP: if (get_oneshot_mods() & MOD_MASK_GUI) register_code(KC_LGUI); tap_key(mouse_key); break;
    case DOUBLE_SINGLE_TAP: register_code(KC_LGUI); break;
    case TRIPLE_TAP:
    case DOUBLE_HOLD: if (get_oneshot_mods() & MOD_MASK_GUI) { clear_oneshot_mods(); unregister_code(KC_LGUI); register_code(KC_LALT); }; register_code(mouse_key); break;
    case DOUBLE_TAP: tap_key(mouse_key); tap_key(mouse_key); break;
  }
}

void mousekey_reset (qk_tap_dance_state_t *state, void *user_data) {
  uint16_t mouse_key = KC_MS_BTN1;
  int layer = -1;

  if (user_data) {
    qk_tap_dance_pair_t *pair = (qk_tap_dance_pair_t *)user_data;
    mouse_key = pair->kc1;
    layer = pair->kc2;
  }

  switch (mtap_state.state) {
    case SINGLE_HOLD: if (layer > -1) layer_off(layer); else unregister_code(KC_LGUI); break;
    case DOUBLE_SINGLE_TAP: unregister_code(KC_LGUI); break;
    case DOUBLE_TAP: clear_oneshot_mods(); break;
    case SINGLE_TAP: clear_oneshot_mods();
    case DOUBLE_HOLD: unregister_code(KC_LALT); unregister_code(mouse_key); clear_oneshot_mods(); break;
  }
  mtap_state.state = 0;
}

// Add TD(TD_MB_L1) for a key in your keymap to be a mouse button on tap, layer 1 on hold, double tap for double click and double tap hold to drag
// Note, if you double tap hold to drag with CMD it will switch it to ALT for OSX Finder