Extracting JSON objects in Athena / Presto for dynamic keys

I have two string formatted columns in my table. One of the columns has json objects in it. The keys in the JSON objects are not fixed - so the problem is I cant use standard json extract functions. Here is how the table looks

I want to flatten this table to something like below

Can anyone please help? Have spent a lot of time using map_values and map_keys but couldnt get it right.

The only closest thing I could come up with was this

select
   timestamp,
    t.*
FROM mytable
   CROSS JOIN UNNEST(map_entries(CAST(json_extract(upstream_tables, '$') AS MAP(VARCHAR, VARCHAR)))) AS t

@martin-traverso's answer can be used with Athena engine v. 3 which is based on Trino, for both v.2 and v.3 the main trick of casting to map you have discovered, I would switch from using json_extract to json_parse (to transform from string to json), skip map_entries (Presto/Trino can unnest maps to key-value pairs, optionally use MAP(VARCHAR, JSON) as target type) and specify column names for unnest result, Presto/Trino can unnest maps to key-value pairs:

WITH data(ts, value) AS (
    VALUES
        (from_iso8601_timestamp('2023-02-02T17:34:55.946Z'), VARCHAR '{"ETL_table_1":true,"ETL_table_2":true}'),
        (from_iso8601_timestamp('2023-02-02T13:30:11.882Z'), VARCHAR '{"ETL_table_3":true}'),
        (from_iso8601_timestamp('2023-02-02T20:12:22.116Z'), VARCHAR '{"ETL_table_4":true,"ETL_table_5":true,"ETL_table_6":false}')

)
select
   ts,
    t.*
FROM data
   CROSS JOIN UNNEST(CAST(json_parse(value) AS MAP(VARCHAR, JSON))) AS t(upstream_tablename, was_completed);