cancel ahead of time?

Thanks to the Xfinity Forums crew for your responsiveness on the boards.

I am planning on canceling my internet-only (along with Flex box I never wanted and never used, but was required to accept to activate Peacock Premium) service right as the contract/promo rate ends (that promo rate is more than 50% higher than the no contract competition...non promo rate after rate ends is double...and no data caps) in December. I have been reading on the forums about many challenges with canceling and with billing continuing or returned devices being charged. I want to ensure I will be not be charged an extra month if it cancels one day past, but want the service to continue until the end of the billing cycle. I was planning on filling out the online form, but have read here of some issues with that option. I'm looking for the sweet spot date and forum to cancel to try to ensure the least hassle.

When I call/chat/reddit/forum rep cancel, can I cancel effective on a future date (like streaming apps) or does it cancel as soon as I reach out and ask to cancel?

Also, I've read many post that offer directions to communicate directly with support through a chat/support button in the upper right hand corner. I do not see that option. Any guidance would be appreciated.

Thanks in advance.