" The screen is an IPS panel, which reportedly, is not as good as a VA panel for handling contrast. This is a statement that I have been reading for years, so, I would imagine that IPS panels have been improved somewhat over those years. They do have a better wide viewing angle than VA does."

The Panasonic 75" sized LCD models were VA. It is possible Panasonic changed that at some point but I don't see why they would.

Although the native contrast ratio is 3x higher on VA panels that does not automatically mean a VA panel TV is always going to be better in a dark room as software choices can make a VA TV's blacks greyer than an IPS TV's. A well managed IPS panel is much preferable to a poorly managed VA. 

"I have had nothing but problems with Panasonic "

Anything JX that runs "Android TV" is not a real Panasonic but something Panasonic New Zealand has rebadged - probably a TCL as I saw TCL demo models with the same horrific motion (mis)handling as the JX600 and JX700. I hope they learned their lessen and stop rebadging.

"I went and had a look at the X90J in-store today and the JX900, after seeing the Sony I've also ruled out buying the Panasonic.  The X90J really popped out and was way more vibrant, at least in the shop."

The unaltered "Standard" modes on Sonys can be well overcooked and the "Vivid" used in default shop mode is on a whole other level. What you see in a shop often isn't a good representation of what realism you can get out of them at home. How much LCD TVs pop in shops under bright lights is usually not about their real home environment picture quality potential but how much they oversaturate, overinterpolate and overbrighten in their default modes.