Unusable Macbook Pro when connected to external 4K LG Monitor

<<\ So it looks like the USB-C connection to the external monitor is the issue.>>

There has been a LOT of discussion about external displays connected to this specific last-of- its-kind Intel MacBook Pro. You idea that the USB-C is the culprit is misplaced, and using HDMI instead will make things run HOTTER.

When running only the built-in display, that Mac runs on the Integrated Intel graphics processor, and AND its internal Display RAM in low power, lower speed mode. Fan speeds generally stay low.

When used with any external display, the Discrete graphics processor takes over and runs all displays. If the external displays are few in number and under 2K in resolution, users find that one or sometimes two can be used without invoking High graphics chip power AND High RAM speed and power.

16-in 2019 intel MacBook Pro heat and performance:

This computer was built with a ninth generation 14nm Intel processor. The only way to meet the target performance was to make it a six or eight core processor. This processor generates an enormous burst of heat when it does Turbo Boost, and there is only one cooling rail shared by both the CPU and GPU. So getting Either side too hot ramps up the fans.

This processor was supposed to be an eleventh-generation 7nm processor, but intel was three years late, and has only starting to ship its tenth generation 10nm processors recently.

The drive in this computer is more than 100 times the typical speed of drives in computers a decade older. If you have installed software that wastes computer resources on a regular basis, such as third-party Virus Scanners, speeder-uppers, Cleaner-uppers/Removers, Optimizers, third-party file Sync-ers such as DropBox, OneDrive, or GoogleDrive, or a VPN that you installed yourself, it will do busywork at previously-impossible speeds — heating up at a ferocious rate.

This older junky software used to run as fast as it could, then would then have to suspend itself to wait for the disk drive to catch up before continuing to waste resources. With a really fast drive, that drive-speed restriction is gone.

Apple DID redesign the cooling system for this specific Mac. New, high-efficiency fans are used. But for some uses, that is still not enough.

Some of the best practical advice is to install Turbo Boost Switcher and turn off CPU Turbo Boost. This reduces the huge sudden overheating brought on by Turbo Boost, that simply does not translate into real-world performance gains.

For external displays, changing away from HDMI to DisplayPort family (or to DisplayPort over ThunderBolt or USB-C) will reduce the Voltages and slightly reduce the heat generated. It is a good thing to do, but it may not be enough to solve this issue by itself.

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues. This requires a hardware rasterizer/display-generator for each fully-accelerated display. 

Fetching screen data is done by rasterizer/display-generator Hardware in the graphics subsystem. Once set up, data are fetched at that same interval again and again without additional intervention. The entire screen data MUST be completed within each screen refresh interval (often 1/60th second) or the screen will go partially or completely blank without the late or missing data. In addition, each one-pixel-tall Row must be fetched on a much tighter schedule within one row-time, or part of the row will be blank.

On this particular model MacBook Pro, the speed required for large displays and/or multiple external displays often requires its display RAM to run in high speed, high power mode. The extra heat builds up, the cooling rail gets hot, and the fans kick into high speed to unload the heat.

you are already using the least taxing connection method.

the only remaining way to reduce heat generated and fan noise is to download and install a utility like Turbo Boost Switcher.

NOTHING suffers as badly as the last Intel MacBook Pro 16-in.

That is partly because older Macs are also not quite as capable.