replace values in pandas based on aggregion and condition

Question

I have a dataframe like this:

I want to replace values in col1 with a specific value (ex:with "b"). I should count the records of each group based on col1 and col2. For example count of col1 = a, col2 = t is 3 and col1 = a, col2 = u is 1 .

If the count is greater than 2 then replace the value of col1 with 'b'. For this example, i want to replace all "a" values with "b" where col2 = t.

I tried the below code, but it did not change all of then "a" values with this condition.

import pandas as pd
df = pd.read_excel('c:/test.xlsx')
df.loc[df[(df['col1'] == 'a') & (df['col2'] == 't')].agg("count")["ID"] >2, 'col1'] = 'b'

I want this result: