Snowflake SQL - while loop inserting values to the table with statistics and aggregations

I'm trying to create a way of inserting aggregated data into a table by using a loop with a moving cursor. I want to count records in a 1h timeframe and compare it to the average number in corresponding timeframes from the last 7 days. I would set a start of a timeframe as a variable and move it 15 minutes with each loop.

Here is what I've tried so far:

I created the table:

CREATE OR REPLACE TABLE WHILE_LOOP_count_TEST__RESULTS (
  ID int NULL,
  START_TIME TIMESTAMP_NTZ NULL,
  END_TIME TIMESTAMP_NTZ NULL,
  ID_COUNT int NULL,
  PREV_7D_AVG int NULL,
  
  DT DATETIME NULL,
  "CREATE_DT" TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP(),
  "CREATE_USER" VARCHAR2(50) NOT NULL DEFAULT CURRENT_USER()
);
-- START VALUE
INSERT INTO WHILE_LOOP_count_TEST__RESULTS (ID, ID_Count, DT) VALUES (1, 1, '2025-02-15 12:00');

Then, I tried the following query:

EXECUTE IMMEDIATE $$
DECLARE
  counter INTEGER := 1;
  start_time TIMESTAMP_NTZ := '2025-02-19 14:30:20.706000000Z'::TIMESTAMP_NTZ;
  end_time TIMESTAMP_NTZ := DATEADD(hour, 1, start_time);
BEGIN
  WHILE (counter < 5) DO
    -- INSERT 
    INSERT INTO WHILE_LOOP_count_TEST__RESULTS (ID, ID_COUNT, DT, PREV_7D_AVG)
    SELECT ID, ID_COUNT, DT, PREV_7D_AVG
    FROM (
      ((SELECT MAX(ID) + 1 AS ID FROM WHILE_LOOP_count_TEST__RESULTS),
          COUNT(DB1.TRANS_ID) AS ID_COUNT,
          DATEADD(minute, +15, (SELECT MAX(DT) 
        FROM WHILE_LOOP_count_TEST__RESULTS)) AS DT,
      Merchant
      
     FROM
      DB1 
      LEFT JOIN DB2 ON DB2.EXTERNAL_IDENTIFIER = DB1.TRANS_ID
      LEFT JOIN DB3 ON DB2.COUNTRY_CODE = DB3.COUNTRY_CODE
    WHERE
      DB2.TS > :start_time
      AND DB2.TS < :end_time
      AND DB2.TYPE = 'INI'
    GROUP BY
      merchant ,
      DB3.COUNTRY_REGION) as t1
      LEFT JOIN
      (SELECT avg(number) as PREV_7D_AVG, merchant1 from (
                        (
                            (
                                select
                                            count(TRANS_ID) as number,
                                            COUNTRY_REGION as CONTINENT,
                                            merchant1
                                        from
                                            DB1
                                            left join DB2 on external_identifier = trans_id
                                            left join DB3 on DB2.COUNTRY_CODE = DB3.COUNTRY_CODE
                                        where
                                            ts > dateadd(hour, -2, :start_time)
                                            and ts < dateadd(hour, -1, :start_time)
                                            
                                        group by
                                            CONTINENT,
                                            merchant1
                                        having
                                            number > 30
                                    
                            )
                            union all
                            (
                                select
                                            count(TRANS_ID) as number,
                                            COUNTRY_REGION as CONTINENT,
                                            merchant1
                                        from
                                            DB1
                                            left join DB2 on external_identifier = trans_id
                                            left join DB3 on DB2.COUNTRY_CODE = DB3.COUNTRY_CODE
                                        where
                                            ts > dateadd(hour, -26, :start_time)
                                            and ts < dateadd(hour, -25, :start_time)
                                            
                                        group by
                                            CONTINENT,
                                            merchant1
                                        having
                                            number > 30
                            )) 
                            -- then six more unions, every one one day earlier
                        group by
                            merchant1::varchar,
                            CONTINENT

                        
                    )  as t2 ON t1.Merchant = t2.merchant1);

    -- Update the start time for the next iteration
    start_time := DATEADD(minute, 15, start_time);
    end_time := DATEADD(hour, 1, start_time);
    counter := counter + 1;
  END WHILE;
  RETURN counter;
END;
$$;

I feel like I'm close to fixing this problem, but I'm getting either I'm getting either "syntax error line 7 at position 17 unexpected '<'. " or "ncaught exception of type 'STATEMENT_ERROR' on line 9 at position 4 : Single-row subquery returns more than one row.", the second one if I'm not joining the t2 table but just put the subquery in the main select statement. Any suggestions?