have difficulty trying to accurately find out which shows are definitive ongoing or cancelled.

Especially over the last few years that tends to change a lot more often than it used to. Cancelled shows get brought back, sometimes by the same network, often by a different network. It is legit hard to keep up with, and I doubt there is a 100% reliable source at any given time. That said, I use next-episode.net which mostly pulls info from thetvdb.com It usually gets updated quickly enough that I don't miss too much, but it's by no means 100% accurate.