I have never used the WatchPort sensors. My X4 has 8 Xbee devices (AIO, DIO, serial) & runs 15% utilization. Some of my devices read every second, others every 15 minutes - but I have them all set to send-in data to the gateway. I don’t think the WatchPort can do that; it must be polled by X4.
One suggestion, if you have newer gateway firmware which supports the over-the-air update of Xbee firmware, turn that off. It can cause “mesh congestion” for many minutes after you reboot the gateway.
Some cellular systems also cause near 100% usage during PPP start-up, so don’t activate cellular unless you need it.
Easiest first test - change all of your poll rates to 15 second, then 60 seconds. See what the effect is.