Well here's an update: I bought a Kawai KDP120 (in black)! Here are some early thoughts in comparison to the Roland FP-30X for anyone in a similar position to me who might stumble across this thread.
Pros: The action is so much smoother and lighter. It doesn't have that feeling of inertia that the FP-30X does. My hands are sailing across the keys.
As an object in the room, it looks so classy. It's definitely a cheaper unit as far as furniture pianos go, but they've managed to make it look very beautiful indeed. Man, I love having a sliding keyboard cover too.
The volume slider is such a better way to control volume vs mashing an up or down button. Having five LEDs to depict master volume is truly terrible. Roland got that very wrong.
The built in lesson modes are a really cute touch, it's actually quite an impressive selection (Alfred, Czerny, Burgmuller et al) for an adult beginner such as myself, going back to basics and learning it properly rather than just brute force memorising pieces beyond their station.
Neutral: Both are a bit of a pig to control. A clunky bluetooth app or a cryptic key code that you need to cheatsheet or memorise - yuck. But that's just the price you pay at this price point. Roland have the edge here for at least labelling a handful of convenient options, but perhaps Kawai wins it back because it looks less messy without the labels.
Cons: The speakers on the FP-30X seem much better to my ears. Both downward firing which is inherently bad imo. The sound is a little muddy on the Kawai. Maybe some would call this warm, but it feels like a downgrade to my ears. It certainly doesn't sound bad though.
The onboard sounds on the Roland are MUCH better. They're perfectly usable here but the Harpsichord in particular sounds like a children's toy. If I wasn't using Pianoteq I'd be crying.
I knew this from the start, but it's still sad that I can't pipe my VST back through the piano's speakers. Headphones it is, and maybe this is a good excuse to buy some monitors. I'm happier with the MIDI performance on this keyboard vs the Roland though, it feels a lot more expressive to me. I'm no expert in this area though but I couldn't get happy with the Roland despite much tweaking.
I do have to concede that the Roland has much better and more premium action. Although I'm extremely happy with my choice, I have gained new appreciation for the PHA-4 and I wouldn't recommend against it as strongly as I would have before. It's slow for sure, but I'm realising that it was actually quite nice to play (trills etc aside) and I think a slightly more experienced player or someone with bigger hands would be much happier with it than I was. The Kawai is a step down in some ways but it still feels lovely to me, but I imagine more traditional or seasoned pianists would hate it.
So in conclusion, I definitely made the right choice. I've learned it's all about finding the right action for you and I feel incredibly liberated with this new setup. It has made playing fun again and I feel like I've gained a few months of experience thanks to this action. I would still heartily recommend the Roland FP-30X - it might not have been for me, but for less money you get a superb action (just so long as you're not a "shredder") and some incredible features and connectivity.
Would I buy the Kawai again? Ehhhh probably not. If it exploded or I had the energy to send it back, I'd probably splash out the extra for a Roland with PHA-50 action, purely my opinion but it's the best action I've ever tried and if my time with the FP-30X has taught me anything it's that Roland really do get a lot of things right and you actually get a lot of bang for your buck in terms of quality, features and connectivity. That said, their PHA-50 pianos are all a BIG price jump for something that I haven't quite got the chops for yet, so I can't really justify spending the extra grand. Give it a year or two and I'll probably sell this and upgrade, but I'm not feeling negative about the Kawai at all and I'm definitely in a much happier place than before.
Hope this mini review helps someone - and thanks to everyone in this thread for their guidance!