SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, descr1.product
  ) as product, 
  1 as instock_marker, 
  IF(
    IF(
      products.tracking = 'O', 
      (
        SELECT 
          MAX(amount) 
        FROM 
          cscart_product_options_inventory s_inventory 
        WHERE 
          s_inventory.product_id = products.product_id
      ) > 0, 
      products.amount > 0
    ), 
    1, 
    IF(products.tracking = 'D', 1, 0)
  ) as instock_sorting 
FROM 
  cscart_products as products 
  LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
  AND descr1.lang_code = 'ru' 
  LEFT JOIN cscart_product_prices as prices ON prices.product_id = products.product_id 
  AND prices.lower_limit = 1 
  INNER JOIN cscart_products_categories as products_categories ON products_categories.product_id = products.product_id 
  INNER JOIN cscart_categories ON cscart_categories.category_id = products_categories.category_id 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_ult_product_descriptions shared_descr ON shared_descr.product_id = products.product_id 
  AND shared_descr.company_id = 1 
  AND shared_descr.lang_code = 'ru' 
  LEFT JOIN cscart_product_options_inventory as inventory_b ON inventory_b.product_id = products.product_id 
  AND inventory_b.amount >= 1 
WHERE 
  1 
  AND cscart_categories.category_id IN (203) 
  AND products.product_id NOT IN (287) 
  AND cscart_categories.company_id = 1 
  AND (
    products.usergroup_ids = '' 
    OR FIND_IN_SET(0, products.usergroup_ids) 
    OR FIND_IN_SET(1, products.usergroup_ids)
  ) 
  AND products.status IN ('A') 
  AND prices.usergroup_id IN (0, 0, 1) 
  AND (
    IF(
      products.tracking = 'O', inventory_b.amount >= 1, 
      products.amount >= 1
    ) 
    OR (products.tracking = 'D')
  ) 
GROUP BY 
  products.product_id 
ORDER BY 
  instock_sorting DESC, 
  products.amount desc, 
  products.product_id ASC 
LIMIT 
  0, 6

Query time 0.00187

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "if(if(products.tracking = 'O',(subquery#2) > 0,products.amount > 0),1,if(products.tracking = 'D',1,0)) desc, products.amount desc, products.product_id",
      "temporary_table": {
        "table": {
          "table_name": "inventory_b",
          "access_type": "system",
          "possible_keys": ["pc"],
          "rows": 0,
          "filtered": 0,
          "const_row_not_found": true
        },
        "table": {
          "table_name": "cscart_categories",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["category_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        },
        "table": {
          "table_name": "products_categories",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "pt"],
          "key": "PRIMARY",
          "key_length": "6",
          "used_key_parts": ["category_id", "product_id"],
          "rows": 8,
          "filtered": 100,
          "attached_condition": "products_categories.category_id = 203 and products_categories.product_id <> 287",
          "using_index": true
        },
        "table": {
          "table_name": "products",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "status"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["orbit_cscart_orbit_cscart.products_categories.product_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "(products.usergroup_ids = '' or find_in_set(0,products.usergroup_ids) or find_in_set(1,products.usergroup_ids)) and products.`status` = 'A' and (if(products.tracking = 'O',<cache>(NULL >= 1),products.amount >= 1) or products.tracking = 'D')"
        },
        "table": {
          "table_name": "shared_descr",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "product_id", "company_id"],
          "key": "PRIMARY",
          "key_length": "13",
          "used_key_parts": ["product_id", "lang_code", "company_id"],
          "ref": [
            "orbit_cscart_orbit_cscart.products_categories.product_id",
            "const",
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(shared_descr.lang_code = 'ru')"
        },
        "table": {
          "table_name": "descr1",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["product_id", "lang_code"],
          "ref": [
            "orbit_cscart_orbit_cscart.products_categories.product_id",
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(descr1.lang_code = 'ru')"
        },
        "table": {
          "table_name": "prices",
          "access_type": "ref",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id"
          ],
          "key": "usergroup",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["orbit_cscart_orbit_cscart.products_categories.product_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "prices.lower_limit = 1 and prices.usergroup_id in (0,0,1)",
          "using_index": true
        },
        "subqueries": [
          {
            "query_block": {
              "select_id": 2,
              "table": {
                "message": "Impossible WHERE noticed after reading const tables"
              }
            }
          }
        ]
      }
    }
  }
}