Improve SQL Server query to convert arbitrary table to JSON

declare @xd table (col1 varchar(max), col2 int, col3 real, colDate datetime, colNull int);

declare @i int = 0;

while @i < 10000 begin
    set @i += 1;
    insert into @xd
    select '', null, null, null, null
    union all select 'ItemA', 123, 123.123, getDate(), null
    union all select 'ItemB', 456, 456.456, getDate(), null
    union all select '7890', 789, 789.789, getDate(), null;
end;

select *
into #json_base
from (
    -- Insert SQL Statement here
    select * from @xd
) t;

declare @columns table (
    id int identity primary key,
    name sysname,
    datatype sysname,
    is_number bit,
    is_date bit);

insert into @columns(name, datatype, is_number, is_date)
select columns.name, types.name,
       case when number_types.name is not NULL
            then 1 else 0
       end as is_number,
       case when date_types.name is not NULL
            then 1 else 0
       end as is_date
from tempdb.sys.columns
join tempdb.sys.types
    on (columns.system_type_id = types.system_type_id)
left join (values ('int'), ('real'), ('numeric'),
                  ('decimal'), ('bigint'), ('tinyint')) as number_types(name)
    on (types.name = number_types.name)
left join (values ('date'), ('datetime'), ('datetime2'),
                  ('smalldatetime'), ('time'), ('datetimeoffset')) as date_types(name)
    on (types.name = date_types.name)
where object_id = OBJECT_ID('tempdb..#json_base');

declare @field_list varchar(max) = STUFF((
    select '+'',''+' + QUOTENAME(QUOTENAME(name, '"') + ':', '''')
           + '+' + case when is_number = 1
                        then 'COALESCE(LTRIM('
                                + QUOTENAME(name) + '),''null'')'
                        when is_date = 1
                        then 'COALESCE(QUOTENAME(LTRIM(convert(varchar(max), '
                                + QUOTENAME(name) + ', 126)),''"''),''null'')'
                        else 'COALESCE(QUOTENAME('
                                + QUOTENAME(name) + ',''"''),''null'')'
                   end
    from @columns
    for xml path('')),
    1, 5, '');

create table #json_result (
    id int identity primary key,
    line varchar(max));

declare @sql varchar(max) = REPLACE(
    'insert into #json_result '
  + 'select '',{''+{f}+''}'' '
  + 'from #json_base', '{f}', @field_list);

exec(@sql);

update #json_result
set line = STUFF(line, 1, 1, '')
where id = 1;

select '['
UNION ALL
select line
from #json_result
UNION ALL
select ']';

drop table #json_base;
drop table #json_result;