Help I've got a issue with Dual Monitors on my 2023 Mac Pro M2 Ultra

there is no special connector for HDMI 2.1. It should be completely indistinguishable in every way, except for a tag attached to the cable.indicating it is certified.

There is a High Retenion Force connector on some cables, that does not follow the advanced protocols -- any cable could have this connector. it has a very slightly different look, but should be completely interchangeable as well. The places where the extra hooks go

... is reserved for exactly this purpose.

<<. there's absolutely no DisplayPorts 1.4 on my Mac. >>

correct as written. No DIRECT displayPorts. however, DisplayPort protocol can be adapted from any of the ThunderBolt ports on your display cards or on the chassis.

The usual way to obtain a DisplayPort connection is with a ThunderBolt-to-DisplayPort adapter/cable like this one:

Monoprice USB-C to DisplayPort Cable - 32.4Gbps Active 8K60Hz Thunderbolt 4/3 Compatible, Black, 3ft Product # 24443 - $11.99

...

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"Premium High Speed HDMI cable" or that + "with Ethernet" --OR--

"Ultra High Speed HDMI cable" or that + "48G"

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

I have blithely assumed your issues are completely limited to your Sony Bravia TV display. (I think I use that same model TV as my main home TV set, and it works well for that use.)

is that correct? any issues with the other display?

On your main screen, running data rates that high runs smack into the limits of switching speeds for electronic devices. cable length too long cause transmission errors, and the display can flicker or blink completely off. This means cable lengths are limited to one meter from the computer to the DisplayPort adapter electronics.

In most cables that is the total length of cable allowed, but the ACTUAL limit, if you had a cable that looked like a snake that just had lunch, is one meter TO the converter electronics, and one meter after the converter to the display.

if you limit your HDMI display to strictly HDMI 2.0 speeds it uses the same signaling method used by EVERY HDMI connection, but tops out at around 4K at 60 Hz.

Once you exceed that limit, EVERYTHING changes to use HDMI 2.1 rules. The Signals in the cable are re-assigned, the encoding method is different and if you don't have "ULTRA" rated cables, it does not work at all.

But the HDMI authorities are quiet when it comes to maximum cable length. Every other interface running at these high speeds, and using that modulation technique (admittedly HDMI is using higher Voltages, but the same signaling) is limited to one meter of cable. I can't cite an authority that says it MUST be that way, but it would be prudent to reduce your cable length when using HDMI 2.1 to one meter or less.

I would test the shortest and best HDMI to HDMI cable that I had connected to the 43" ASUS.

Or optionally get a short high speed USB-C/TB to DisplayPort cable to connect the 43" ASUS.

for example > https://www.amazon.com/dp/B0CSJQY34N/ref=twister_B0CPFKZ5KH?_encoding=UTF8&th=1

Then I would test the USB-C/TB to HDMI cable that you used with the 2019 connected to the 55" Sony TV.

Or optionally test the best 3 meter HDMI to HDMI cable that I had connected the 55" Sony TV.

for your main display is 4K with HDR 10-bit capability.

I/O PORTS

DisplayPort 1.4 DSC

x 2

HDMI (v2.0)

x 2

using ONE DisplayPort adapter/cable limited to 1 meter length, that can support 4K at 96Hz. The date rate HBR3, a very fast 25.92 G bits/sec, and faster than MOST Mac USB port,s but not faster than the ThunderBolt ports on the Mac Pro 2023.

using ONE HDMI 2.0 premium rated cable of longer length, you can get 6K at 50 Hz, before the data format is downgraded and dithering is used to make uptake shortfall in bandwidth. This is operating a t 14.4 G bits/sec.

One HDMI 2.1 cable switches to a different data encoding and clocking, must be ULTRA/48G rated,, and runs at 42,5 G bits/sec with no compromises EXCEPT the suggestion (not statement) that length may be limited.

...and now for something completely different.

You could connect TWO adapter cables to your PIcture-in-Picture capable display, and each HALF display Left/Right could easily run at more modest speeds on its own cable. In the Mac, you set it up as two displays, then ARRANGE them adjacent Left/Right. in the Display, you use Picture-By-Picture (cousin of Picture-in-Picture) to place the two halves back in their proper orientation.

Readers report this works very nicely in Macs.

Apple does not recommend using HDMI adapters, especially at highest data rates, so the recommendation would be to use two ThunderBolt-to-DisplayPort adapter cables. Since each would be driving a 'very tall' 2K display, the restrictions are substantially reduced. the data rates for DisplayPort would drop to HBR2 speeds of about 17.28 G bits/sec for each of two cables, assuming 10 bits/pixel HDR.

<< cable adapters were a very bad idea as they over heat. >>

There are words musing from that you wrote.

cable adapters TO HDMI were a very bad idea as they over heat.

(and that is the main reason Apple does not recommend them. The other is that HDMI is so confusing, and there are no bright lines or nomenclature changes when the technology and the requirements change.)