All I'm spreading around is my personal experience and observations. I stand by my comments. All Mac notebooks with dGPU are forced to use the dGPU when an external display is connected. The 16" MBP may also have other issues as you have described, or more severe symptoms from the same underlying issues. But the point remains that even if you are not doing anything particularly intensive with an external display and the lid open, you will still give up some available power and thermal headroom for the dGPU (5-20W), which is therefore no longer available for the CPU. And this won't change until Apple changes the hardware requirement to use dGPU for external displays, which seems unlikely.
I've spent 2 months and 40+ hours dealing with Apple support over this issue on a 2019 15". It's been tested by Apple twice and the results came back both times "working as designed". This machine uses ~18W for "Radeon High Side" with a 27" 4K display at "looks like 2560x1440" resolution attached and the lid open (via USB-C->USB-C, TB3->DP, and DP->DB via TB3 dock). It uses ~12W for all of the following: "looks like 3008x1692" (text and UI too small), "default for display" (text and UI too big), "low resolution" 2560x1440, and another native 2560x1440 display. It uses ~5W with just the internal display and the dGPU force enabled.
With 18W used for "Radeon High Side" it is pretty easy to make this machine start freezing up for 10s at a time. At the extreme, Intel Power Gadget report frequency drops to 800MHz and Power/PKG drops as low as 7W under load (when it should be sustaining ~40-50W), even while CPU temps are ~70-80c. This happens in Mojave and Catalina. It happens on a fresh install of macOS with zero software besides Safari (playing a 1080p YouTube video) and additionally exercising the dGPU by triggering the expose animation repetitively with the external display attached (Apple insisted I reproduce the issue without any other software, and without any artificial stress tests). It also happens at other resolutions, but requires a little more exercising of the dGPU to trigger.
I recommend that anyone with a 15" or 16" MBP connect an external 4K monitor at various resolutions (default, "looks like" 3008x1692/2560x1440, and "low-resolution" 2560x1440) and run Intel Power Gadget to watch Power/PKG and Frequency graphs while running Cinebench on loop (via min test duration) and Luxmark GPU stress test (which loops indefinitely) at the same time to see how dGPU use can completely cripple the CPU. Granted this is an artificial stress test, but I only ever needed to run it to try and reliably reproduce an issue (freezing system) that was happening frequently under normal use (85% idle CPU, normal apps e.g. browsers and code editors and terminal etc. running but idle during a long video conference call). Run the same Cinebench test with only mild dGPU usage (1-1 video chat on Discord or Zoom) or repetitively triggering the expose animation, and then both scenarios with just the internal display to compare dGPU usage (light and extreme) affects CPU performance.
If you don't see any degradation of CPU performance in a 2019 15" MBP with external 4K display, I'd love to hear about it.
My iMac by comparison can run this same stress test indefinitely with a stable ~4GHz frequency. 15" and 16" MBPs just don't have enough thermal headroom for CPU *AND* dGPU, and you're FORCED to use dGPU with an external display.
The 16" may also have even more extreme problems such as 18-20W Radeon High Side at *all* resolutions with the lid open, but that doesn't mean older 15" models weren't being CPU limited under normal/non-graphics-intensive workloads with external monitors.
So I still recommend waiting for the updated 13/14" which is about to drop and hopefully has a good CPU/iGPU bump and 32GB, if you can live with 4 cores. If you *need* 6/8 cores in a portable machine, I'd be looking at a Razer Core X eGPU. A noisy or even a noisy MBP doesn't bother me. One that runs at 800MHz does. And I'm sure you can swap out the eGPU chassis fan for a quiet Noctua and get a graphics card with quiet fans. Many don't even run the fans at all at low temps.
A refreshed 16" or updated drivers may or may not solve some of the additional problems/symptoms you mentioned in the 2019 16", but I doubt it will resolve the fact that MBPs with dGPUs are struggling to utilise CPU and dGPU at the same time, and you're forced to use the dGPU with external displays.
No matter what apps or benchmarks I run to tax both CPU and dGPU, these machines should have sufficient cooling to run the CPU at the advertised base frequency, at a minimum. They can't. That's not new, and I think it likely won't be fixed.
But by all means, I am all for people engaging with Apple support to get the number of complaints up. Email Tim Cook if you don't get anywhere with second level support. This is definitely a problem that should be resolved. I just don't have faith that it will given the effort that I and others have reported in troubleshooting with Apple directly, and the fact that forced use of dGPU has always been the case and always generated complaints, from what I've read.
