External Monitor Flickering/Ghosting, USB-C, Thunderbolt, Displayport, Macbook M4 Max

Just so that you completely understand, when connecting ThunderBolt-3 or -4 devices, those cables are NO DIFFERENT than "regular" Thunderbolt-3 cables. Their internal wiring and construction are likely to be identical.

The way they achieve faster speeds with ThunderBolt-5 devices at each end is by using a modulated signal -- that sends multiple bits per signaling interval.

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when I said use HDR=OFF, that setting is at The MAC end, in settings > displays. That produces an 8 bits/color data stream that is slightly less 'near the edge'.

<< (although isn't USB-C just encapsulating the DP signal?). >>

It's only trivially more complex than that, so YES.

<< Any suggestions for changes in the apple configuration to avoid this flickering? >>

Dropping back to HDR=OFF (which gives you "only" Millions of colors instead of Billions) will pull you back from the bleeding-edge where this stuff seems to be falling apart on you. For most ordinary purposes short of magazine-cover photo creation, this choice many not even be noticeable.

That Dell U2720Q display appears to be a 4K display limited to 60 Hz refresh that supports Billions of colors when HDR=ON, 10 bits/color

Interfaces

• DisplayPort (DisplayPort 1.4 mode)
• HDMI (HDCP)
• USB-C

There is NO ThunderBolt on that list.

• ThunderBolt cables alone should perform as expensive USB-C cables as long as under 1 meter.

• That display can NOT be used as the FIRST display on a daisy-chain (on a Mac) withOUT using an External Docking device.

re: HDMI

There is a very UGLY knee in the curve of HDMI performance at the nominal 4K @ 60 (8 bits/color) point.

• HDMI 2.0 supports this (4K@60 at 8 Bits) resolution, or 10 bits/color, but only at 50Hz all using PREMIUM cables

• HDMI 2.1 uses a completely different signaling method, and WILL NOT work reliably on any cables that are not at least ULTRA rated. (Thankfully those ULTRA cables are backward-compatible.)

Punchline:

It appears this Display does NOT support higher than HDMI 2.0, and if you want 60 Hz, no higher than 8 bits/color.

The following article contains information about using an Apple Mac computer with a Dell UltraSharp Monitor using a USB-C cable.

Using a Dell UltraSharp USB-C Monitor with a Mac

You must download from Dell the Mac OS driver software for this monitor to get all the available color features. Response from Dell. Dear Valued Customer ...

Dell UltraSharp 27 4K Thunderbolt Hub Monitor - U2725QE

That Dell U2720QE appears to be a 4K display with support for 10 Bits color when HDR=ON. Although nominally supports refresh rates higher than 60 Hz, the Dell article linked above suggests USB-C and ThunderBolt connections (To a MAC) are limited to 60 Hz (it does not say, but may also be limited to 8 bits/color, HDR=OFF).

Connectivity 

• 1 DisplayPort 1.4 port with DSC support (DRR for Microsoft Windows)
• 1 DisplayPort 1.4 port out 
• 1 HDMI port (Supports up to UHD 3840 x 2160 120Hz FRL, VRR as per specified in HDMI 2.1)
• 1 Thunderbolt 4 upstream port (DP1.4 (HDCP 2.2) with DSC support, PD: Up to 140W EPR , 2/4 Lanes switching)

and also:

• 1 Thunderbolt 4 downstream port (Downstream, Daisy Chain, 15 W)
• 1 USB-C (USB 10Gbps KVM) upstream port >> N.B. NOT fast enough for ONE 4K Video (except 30 Hz at 8 bits/color)

This display does Not suffer the same limitations on its HDMI inputs, but requires an HDMI ULTRA cable for most uses.

in every case, cables carrying USB-C or DisplayPort signals are limited to 1 meter maximum length for reliable operation.

I can't see your equipment, and have no idea about the dozens of other choices you have selected.

can you please complete the picture?

exactly what cables, adapters and docks are you using to make each and every connection?