It's slightly more complicated than that, even.
The 5.1 JP track on the UK 4K disc is different to the 192kHz JP track on the JP 4K disc. The UK track is mixed differently, sounds muddy, mixed towards the rears and the mids and vocals sound recessed. It's a bit of a disaster and I really don't know how it happened. The JP track is undoubtedly better.
However...
The 192kHz JP track is also mixed differently to the 192kHz track on the standard BD (both UK and JP), and the BD mix is the better version to my ears. I understand that the JP 192kHz 4K track is a newer mix, and some people may prefer it to the BD mix.
If I have to choose, my personal favourite version on the 4K disc is the JP 2.0 mix, which is only on the JP disc - again, why? My favourite version overall is the JP/UK BD 192kHz which is obviously not available with a 4K HDR picture.
God's honest truth, I was much happier before this whole 4K debacle. Knowing that the best picture and best audio are on two different versions of the film has pretty much killed my enthusiasm to watch it again at this point.
After a bit of research and personal listening, I've come to pretty much the same conclusion as you.
There are only actually 2 5.1 JP mixes, (2009 BD and 2019 UHD), however they've made their way out to the mass market in 3 iterations here.
The issue with the UK/US UHD JP track is it was sampled down (for what reason I couldn't tell you) to 48khz from the 192khz original. This has somehow totally screwed with it and it sounds awful. The JP UHD is the same 2019 JP mix, but it hasn't been f**ked about with and sounds much better.
I'm not 100% on this, but I think the JP track on the new BD (that's included with the UHD) is actually the 2019 mix, and therefore the same track that's on the JP UHD.
To my ears the 2009 mix on the old BD sounds the best, and that's probably the best overall disc to watch, but I'm yet to try the Geman UHD.
The German UHD disc does have a 192khz 5.1 JP track in a DTS HD MA container. So that should atleast put it on par with the JP UHD. From the track stats on bdchecker it appears to have more in common with the older 2009 mix than the newer one though, but I haven't bought it yet to confirm.
The 48khz JP track on the UK/US UHD is awful.
The 192khz JP track on the JP UHD is better.
The 192khz JP on the old BD is easily the best.
The 192kHz JP on the DE UHD - unconfirmed.