Chudnovsky algorithm python incorrect decimals - python-3.x

Question

my goal is to get 100.000 or 200.000 correct decimals of Pi in Python. For this, I have tried using the Chudnovsky algorithm, but I've got some issues along the way.
First, the program only gives me 29 chars, instead of the 50 I want to test the correctness. I know this is a small issue, but I don't understand what I've done wrong.
Second, only the first 14 decimals are correct. After those, I start getting inaccurate Pi decimals according to about all Pi numbers on the internet. How do I get way more correct decimals?
And last, how do I let my code run on all 4 of the threads I have? I've tried using Pool, but it doesn't seem to work. (Checked it with Windows task manager)
This is my code:
from math import *
from decimal import Decimal, localcontext
from multiprocessing import Pool
import time
k = 0
s = 0
c = Decimal(426880*sqrt(10005))
if __name__ == '__main__':
start = time.time()
pi = 0
with localcontext() as ctx:
ctx.prec = 50
with Pool(None) as pool:
for k in range(0,500):
m = Decimal((factorial(6 * k)) / (factorial(3 * k) * Decimal((factorial(k) ** 3))))
l = Decimal((545140134 * k) + 13591409)
x = Decimal((-262537412640768000) ** k)
subPi = Decimal(((m*l)/x))
s = s + subPi
print(c*(s**-1))
print(time.time() - start)

Answer 1

In addition to the small details discussed in the comments and proposed by #mark-dickinson I think I've fixed the multithreading but I haven't had a chance to test it, let me know if it works properly
UPDATE: Problems after the 28th digits were due to the assignment of sq and c before the decimal context change. Reassign their value after changing the context precision solved the problem
from math import *
import decimal
from decimal import Decimal, localcontext
from multiprocessing import Pool
import time
k = 0
s = 0
sq = Decimal(10005).sqrt() #useless here
c = Decimal(426880*sq) #useless here
def calculate():
global s, k
for k in range(0,500):
m = Decimal((factorial(6 * k)) / (factorial(3 * k) * Decimal((factorial(k) ** 3))))
l = Decimal((545140134 * k) + 13591409)
x = Decimal((-262537412640768000) ** k)
subPi = Decimal((m*l)/x)
s = s + subPi
print(c*(s**-1))
if __name__ == '__main__':
start = time.time()
pi = 0
decimal.getcontext().prec = 100 #change the precision to increse the result digits
sq = Decimal(10005).sqrt()
c = Decimal(426880*sq)
pool = Pool()
result = pool.apply_async(calculate)
result.get()
print(time.time() - start)