Try to install a downgrade on a separate partition to ensure this isn't a suddenly new hardware issue that cropped up.
How do I downgrade my Mac OS X operating … - Apple Community
If downgrading reveals a new hardware issue, try swapping out better cables.
The 2018 MacBook Pro takes a USB-C power adapter, which can have its USB cable fed into a USB-C hub/docking station like these:
https://www.smklink.com/products/usb-c-100w-mini-docking-station-with-multi-stream-triple-video
https://www.siig.com/usb-c-mst-video-with-hub-lan-pd-3-0-dock.html
and the docking station fed directly where you would feed power into your computer.
I would not use any Insignia HDMI cables unless you have an insignia display. Many Insignia cables have very poor signal quality, and appear to be best designed for their own displays. Belkin and Tripp Lite are both better brands. And the gold tipped cable are worth the extra cost. Thunderbolt 3 or higher based displays need thunderbolt cables with the lightning bolt icon on the connector, and can't just use any USB-C cable.
