Coming here after the hard challenge, and I realized that the hard challenge already did most of my work for me, so here's the solution with help from the hard challenge. :)

Python: https://pastebin.com/0Neaj0r9

import sys
from itertools import product
 
string_length = sys.argv[1]
 
matches = {
    "}": "{",
    "]": "[",
    ")": "("
}
 
brackets = ['{', '[', '(', ')', ']', '}']
 
 
def bracket_gen(length):
    combinations = product(brackets, repeat=length)
    return [''.join(combo) for combination in combinations]
 
 
def get_matching_substring_strict(string):
    substring = ''
    index_start = -1
    index_end = -1
    bracket_counts = {
        "{": 0,
        "[": 0,
        "(": 0
    }
    for index, letter in enumerate(string):
        if letter in matches.values():
            if index_start == -1:
                index_start = index
            substring += letter
            bracket_counts[letter] += 1
        if letter in matches.keys():
            if not substring:
                break
            if substring[-1] == matches[letter]:
                substring = substring[:-1]
                bracket_counts[matches[letter]] -= 1
                if not [cnt for cnt in bracket_counts.values() if cnt]:
                    index_end = index
                if [cnt for cnt in bracket_counts.values() if cnt < 0]:
                    break
            else:
                break
 
    if index_start != -1 and index_end != -1:
        matching_substring = string[index_start:index_end + 1]
        return matching_substring
 
 
valid_combos = []
bracket_combos = bracket_gen(eval(string_length))
for combo in bracket_combos:
    if combo == get_matching_substring_strict(combo):
        print(combo)