How can I push 4K @ 60Hz from 2016 MacBook Pro?

  Displays:
Color LCD:
  Display Type:	Built-In Retina LCD
  Resolution:	2880 x 1800 Retina
  Framebuffer Depth:	30-Bit Color (ARGB2101010)
  Mirror:	Off
  Online:	Yes
  Automatically Adjust Brightness:	Yes
  Connection Type:	Internal
SwitchResX4 - DELL S2721QS:
  Resolution:	5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
  UI Looks like:	2560 x 1440 @ 30.00Hz
  Framebuffer Depth:	30-Bit Color (ARGB2101010)
  Display Serial Number:	7D16M43     
  Main Display:	Yes
  Mirror:	Off
  Online:	Yes
  Rotation:	Supported
  Automatically Adjust Brightness:	Yes
  Connection Type:	DVI or HDMI
  Adapter Firmware Version:	0.00

When I select "Default for Display", I get

SwitchResX4 - DELL S2721QS:
  Resolution:	7680 x 4320 (4320p/8K UHD 2 - 8K Ultra High Definition)
  UI Looks like:	3840 x 2160 @ 30.00Hz
  Framebuffer Depth:	30-Bit Color (ARGB2101010)
  Display Serial Number:	7D16M43     
  Main Display:	Yes
  Mirror:	Off
  Online:	Yes
  Rotation:	Supported
  Automatically Adjust Brightness:	Yes
  Connection Type:	DVI or HDMI
  Adapter Firmware Version:	0.00

Obviously this "Resolution" field is incorrect - the display only supports up to 3840x2160. How do I manually set the EDID (and figure out what it should be)?

I did, although I have uninstalled it, since my free trial ran out. Here is the result with it fully uninstalled and all custom resolutions removed (/Library/Displays/Contents/Resources/Overrides/ empty):

DELL S2721QS:
  Resolution:	7680 x 4320 (4320p/8K UHD 2 - 8K Ultra High Definition)
  UI Looks like:	3840 x 2160 @ 30.00Hz
  Framebuffer Depth:	30-Bit Color (ARGB2101010)
  Display Serial Number:	7D16M43     
  Main Display:	Yes
  Mirror:	Off
  Online:	Yes
  Rotation:	Supported
  Automatically Adjust Brightness:	Yes
  Connection Type:	DVI or HDMI
  Adapter Firmware Version:	0.00

  Displays:
Color LCD:
  Display Type:	Built-In Retina LCD
  Resolution:	2880 x 1800 Retina
  Framebuffer Depth:	24-Bit Color (ARGB8888)
  Main Display:	Yes
  Mirror:	Off
  Online:	Yes
  Automatically Adjust Brightness:	Yes
  Connection Type:	Internal

The frequency isn't listed, but I believe it's 60 Hz (as validated by online tests). Only the new MBPs have 120Hz.

Yeah, that is interesting. Also to note, when the external display is connected, both displays run off the discrete GPU (Radeon Pro 450), while when none are connected the display runs off the integrated Intel HD Graphics 530.

I remember that a while ago, I installed custom AMD graphics drivers on my Bootcamp partition to enable better support for my previous monitor (Dell U2520D), and I also installed Dell drivers - I don't remember which one fixed my issues (I also don't remember exactly what the issues I was experiencing were).

I don't think these sort of custom drivers exist for macOS itself though, since they should be supported out of the gate.

One other thing that may be relevant is the color mode - Windows runs by default in RGB, while macOS runs in YPbPb. Enabling RGB gives a pink/green hue, since the EDID is wrong. I've messed around a little bit with manually patching the EDID but it ended up being a huge hassle (why does it have to be encoded in base64???).

I'm running macOS Monterey 12.0 beta 9 (21A5543b).

I still don't know what the issue is over HDMI, but I ended up buying a DisplayPort to USB-C cable, and I'm able to get 4K @ 60Hz without any tinkering.

60Hz and 30Hz are definitely very different, Windows is noticeably smoother scrolling and moving the mouse around.

The panel supports 30 bit colors (10 bit RGB/YPbPr panel depth).