SQL Server 2019

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

DECLARE @test VARCHAR(max) = '123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890'

SELECT   String_agg( value, ',')
FROM     (
                  SELECT   String_agg( value, '') within GROUP ( ORDER BY nt1 ASC) value,
                           nt,
                           1 AS t
                  FROM    (
                                    SELECT   substring(a.b, v.number+1, 1) value,
                                             ntile(18) OVER( ORDER BY (
                                                    SELECT NULL) ) nt,
                                             row_number() OVER (ORDER BY (
                                                    SELECT NULL) ) nt1
                                    FROM     (
                                                    SELECT @test b) a
                                    JOIN     master..spt_values v
                                    ON       v.number < len(a.b)
                                    WHERE    v.type = 'P') c
                  GROUP BY nt ) d
GROUP BY t

(No column name)
1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890