we have hit the brick wall.

Bill Claff does. He just makes you search for it, because he's so fond of his, in my opinion, useless PDR metric. If you go to his site and choose the 'input referred read noise' chart.

Then select the camera(s) you're interested in, and you get the read noise chart.

Click on the name of a camera (red arrow) and you get the DR (as Bill insists, the 'EDR') data.

For comparison, here's the Mk III

There's about half a stop DR difference - probably the result of faster ADC converters (which are usually noisier).
Alternatively there is still DxOMark.
As to how to interpret the data, first, this is un-normalised - it is at the native resolution. To normalise it to some required standard resolution you need to add half of the log-2 pixel ratio. So, for instance if you want to normalise at 8MP (A3 at 300ppi) then you need to add log2(24/8)/2 = 0.8. My preferred normalisation value is 24MP (A3 at 360 ppi) - since this I think reflects the needs of high-end photographers. Then you need to decide what is the acceptable low end, remembering that this will always be the deepest shadows in your final image (since you'll never be pushing shadows to light grey). I've done some experiments and I think an SNR of 2:1 is fine in the deepest shadows (remember they're Zone I, defined as 'near black with slight tonality but no texture'). That would subtract a stop from the raw DR. So that would give you 11.7 stops at 24MP for the Z6 III 100 ISO and 12.2 stops for the Z6 II. If you chose figures for 8MP (A3 print) it would go to 12.5 and 13 stops.
8MP also corresponds pretty much to 4k video. If you wanted display on an HD screen, about 2k then you can add another stop.
It's also worth remembering that you'll only get that DR if you expose right up to the highlights in raw, which will usually mean an exposure somewhat above the nominal ISO exposure indicated by the meter. If you just expose according to the meter, and want to know how much you can push the shadows, then you also need to know how much raw headroom there is, which only so far comes from DxOmark.

Just another pet peeve here I'm reminded of doing these screen shots - the way Bill advertises it as 'your trusted source' - it's not up to him to decide whether it's 'trusted' or not - that's up to users.

One of the confusions about DR is equivocation - the same word being used to mean different things. There are two main definitions of DR common in photography. One is the one you're giving, which is more properly 'tonal range'. The other is to do with information theory - what is the range of information available in the raw file. You can't really talk about 'clipped blacks' or 'clipped whites' in a raw file because until it's processed the black and white levels have not been set. The pixel saturation gives the highest possible white level, but rarely is white set there in a processed file for various reasons. At the low end the decision on where to set the black level is usually about noise, not hard clipping.

Not my camera, one I'd been sent the raw files for from AP to do the analysis. On the 17 stops, first remember that was an old camera - before the dual conversion gain was being used. Dual CV usually reduces the maximum DR because it increases the read noise at low ISOs. Its use really is to extend the available DR at higher ISO settings. Also, read noise tends to increase as ADC speed increases, and with cameras going for really fast readout it has increased substantially. That camera (an A7II) had a base ISO read noise less than a quarter of the A7IV
On the DR figure itself, it's not remarkable - remember that many cinema cameras claim that kind of DR - it's not that their sensors have any special juice - and the actual DR figure depends on normalisation, which is vital if you want to compare cameras. The likely reason that my figure is higher than Bill's is the way the read noise is measured. The DR is very dependent on that, because it's the denominator of the fraction. Mine is obtained by measuring a black frame (taken with the lens cap on), whilst Bill's is obtained by curve fitting and extrapolation. It has to be done in that way for him because of the way he gets the data, but a direct reading will clearly be more accurate.