PostgreSql : flatten json array data

From my current query, I obtain this jsonb data :

values: "a1", ["b1", "b2"]

And I want to flatten it on one level only, like this :

values: "a1", "b1", "b2"

Here is a simplfied way to get data in a query (only 2 levels are possible, never more):

SELECT * 
FROM jsonb_array_elements('{"test": ["a1", ["b1", "b2"]]}'::jsonb->'test');

I tried to use jsonb_array_elements but my problem is that : I don’t know if it is a json array or not ! Not expert in SQL, I did not find a way to code something like :

SELECT
    IF (is_json_array(list)) 
        jsonb_array_elements(list)
    ELSE
        list
    ENDIF
FROM jsonb_array_elements('{"test": ["a1", ["b1", "b2"]]}'::jsonb->'test');

For a «zoom out» view of my current data, here is a table-free working test :

with recursive search_key_recursive (jsonlevel) as(
    values ('{"fr": {"WantedKey": "a1", "Sub": [{"WantedKey": ["b1", "b2"]}], "AnotherSub": [{"WantedKey": "c1"}]}}'::jsonb)
    union all
    select 
        case jsonb_typeof(jsonlevel)           
            when 'object' then (jsonb_each(jsonlevel)).value        
            when 'array' then jsonb_array_elements(jsonlevel)   
        end as jsonlevel
    from search_key_recursive where jsonb_typeof(jsonlevel) in ('object', 'array')
)
select search_key_recursive.jsonlevel->'WantedKey'
from search_key_recursive
where jsonlevel ? 'WantedKey';

For after, I will use the result in an insert statement :

INSERT INTO table1 
SELECT 'someText', value 
FROM jsonb_array_elements('{"test": ["a1", "b1", "c1"]}'::jsonb->'test');