[Link: wyrestorm.com]
Can you scale the output to the 2nd zone display and send it over coax as 1080p?

Sadly this is only good for first Gen 4K devices.
Note that there is no indicated support for 10 Bit, 4:2:2, HDR or HDCP 2.2.

“The WyreStorm EXP-CON-4K-DD is an in-line HDMI video scaler that can connect between a screen and a distribution system to scale incoming 4K/30Hz content to 1080p. It can also downmix Dolby Digital 5.1 audio to stereo via the HDMI output or stereo audio output port.”

Affordable in-line scaling solution for installations including 4K and HD displays without compromising source resolution
4K/30p > 1080/24p Downscale - to enable 1080p screens to view 4K sources through a 4K matrix system
1080/24p > 4K/30p Upscale - for connection to 4K screens
Supports resolutions up to 4096x2160p @24/25/30Hz
Full 3D support up to 1080p/24hz
Downmix incoming Dolby Digital to stereo PCM audio where required through HDMI output
Supports up to 6 channels (up to 5.1 surround) of audio encoded in Dolby Digital
Low-latency digital filtering

Can scaling be the answer - from a more capable video scaler? OP wants to send video from 2nd zone output of AVR to a TV. Source and TV specs are not specified, but is another scaler available that will support current source resolutions? If so, once downconverted, will the AVR see the scaler as 4k and not affect the main zone video signal?

Currently a safe assumption would be that the OP needs to support HDR and 10 bit as all current source dives do also.

Broadcast side 12G can do full 4:4:4 4k incl HDR and audio (this is, after all, the standard used to MASTER this stuff). 12G can be done on a single piece of mini coax too.

But,.. no HDCP.

I am not sure about the math that you are using, (4K/60, 4:2:0, 10 Bit, HDR is a little over 13), but as you stated HDCP is the real killer here.
Broadcast masters (AFAIK) are currently 4K/30, 4:2:0, 8 Bit w/HLG (on rare occasions), and from what I have seen in the trade mags, IP is quickly taking over the mastering process (which is under 10G) further limiting this process to a lower bandwidth.

Please note that (A) I am not in the broadcast world and (B) I do not know that bandwidth requirements for HLG

Black magic HDMI to 6G-SDI and 6G-SDI to HDMI Converters
It is close to $350 total but I dont know of much of anything cheaper

[Link: blackmagicdesign.com]

[Link: blackmagicdesign.com]

the fact we arent using high quality coaxial cable for video distribution is a travesty!!!!!!!!!!!!! consumer electronics industry at its absolute worst!!!!! they are absolutely transmitting 4k 60p not sure at what color space, over a single coxial cable up to 300m and are working on the 24g spec. they also have dual and quad llnk systems. triple link coaxial systems in place by the thousands would with todays technology support 36Gbps 72Gbps in the near future. oh my god i dont know why i work myself up over this!! oh well, back to my proposals.......

I'm amazed at what a mess the consumer formats are. If it weren't for HDCP, SDI would be so, so much better than HDMI. (EDID is an issue too, but really, who cares about that). I sure wish the pro AV world could use the broadcast standards.

For those that care, here is a link to the SMPTE doc describing the various formats 12G SDI can carry (all the way up to 8k, uncompressed). And yes, this is the mastering format:
[Link: ieeexplore.ieee.org]

There is some transition to IP, but it only makes sense on the largest of systems (ie matrix routers over 1024x1024 cross-points). These systems also require a large number of SDI gateways, since almost all mastering equipment is SDI. 10Gbps is the IP mastering switch speed requirement, but as you can see, 10Gbps can handle all common production formats, uncompressed. The industry has been slow to adopt this due to the cost, complexity, and reliability issues. By comparison, SDI is cheap, simple and reliable.

... Indeed.

Yes, SDI would have been perfect - for video and audio. The blame goes to Hollywood's lawyers. Just think how easy - and cheap - a matrix switch would be to manufacturer.

However, while most of the responses to the original post offered some worthwhile solutions, they all failed to address the elephant in the room which I brought up in the second post. The response from the OP to my question about the coax he ran was answered with "RG6 Quad". That didn't answer my question, which was about the center conductor. If his "RG6 Quad" has the letters CCS printed anywhere along the outer jacket then good luck getting it to work with most of these solutions. I had a project in Pittsburgh (mansion) where the electrician ran copper-clad steel RG6 Quad and the AV integrator (I was strictly doing lighting and shades) installed Snap (Binary) HDMI-over-Coax devices from the EQ room to 16 TV locations. None of them worked. I grabbed the instructions from one of the boxes where it plainly stated "must use solid copper coax".

Did you fail to let the electrician know the specs that YOU needed? That would have been your job, no?.

No. Read it again. He says, then you quote him, that he was just doing lighting. When the HDMI stuff failed, he picked up the instructions and read that they said solid copper coax.

What source are available today to give you the highest spec for 4K? What is the limitation for the signal coming into your house from the cable company through standard coax? I'm not sure how we can rely on coax coming into the house for these signals when you can't distribute through coax inside the house?

Different compression codecs.
A streaming service can use H.265 coming in. Then their box has to convert it to the lossless HDMI for output, which uses a lot more bandwidth for the same video signal.

This is how some of that discrepancy happens.

Service providers can bring out new boxes every year (if need be), with the latest high efficiency codecs.
A connection standard like HDMI however, takes years to standardize. And years for each revision to update. Then they have to be backward compatible with the older connections, which many times cripples the ability to use new high efficiency codecs. So as we push more & more down that HDMI pipe, the bandwidth needed on the cable, goes up exponentially. Because they are locked into an old codec from decades ago now.

If we suddenly switched to a super high efficient codec for HDMI, the bandwidth requirements would be a fraction of what it is now - but then no older devices would/could work with the new HDMI connection standard.

[edit]:
To answer the other part of your question, it still takes a LOT of bandwidth even with higher codecs. So most any streaming service is going to have compression and loss of picture quality.
The only way to get "the best" picture quality right now, is either thru a UHD bluray disc using a UHD bluray player.
Or download the content, then play it from a drive. This is the case with video games and game consoles. Or movies/shows with some movie server devices.