As you mention you have so many products in your catalog, so i suggest to run the cron at midnight or you can select any other time as per your website's traffic timing.
Below is the cron command you need to set in server, first open your SSH ternimal and login in that, then type command crontab -e then copy below code and paste into the terminal window and save the crontab.
0 0 1 * 0-6 php /your_magento_root/bin/magento index:reindex catalog_category_product
The above cron code will execute at every day 12AM UTC timezone like below :
2020-02-29 00:00:00 UTC
2020-03-01 00:00:00 UTC
2020-03-02 00:00:00 UTC
If above code is working or useful for you then upvote the answer and mark the answer as accepted so that other users can easily identify the answer.