@jon_joy_1999:
If you haven't finished manually creating Tags within Calibre for your 500 text files, this might help you by creating Comments and Tags automatically using a list of the 'Top N Nouns' in each text file, sorted by frequency in descending order:
[GUI Plugin] English Noun Frequency :
https://www.mobileread.com/forums/sho...d.php?t=263684
A typical example for a Factual/Non-fiction book is attached just below.
DaltonST