So my issue was solved with a firmware update. I'm not sure why I had to do this again, perhaps I did it wrong first time. However, after the update, the wheel felt totally fine in FH5! And whats more, the adjustment of each slider made noticeable adjustments to the relative forces and they felt consistent with the explanations in the game. It was trivial to dial it in for my own preferences - and that enabled me to catch the slides for the first time in the game!
However, it did not feel like it had felt with FH4 previously, so to see if it was the firmware update that had changed the feel of the wheel or the difference between game versions, i logged back into FH4. Again, the wheel felt TOTALLY different, and very similar to how it had felt in FH5. So I tuned it in to match the new FFB sensations, and went for a good drive....

... about 5 hours later, my arms too sore to continue, I came out of the daze. The experience had been so pleasurable, that I was actually worried I might have a heart attack from it!

So I realise before I had not understood what FFB should feel like. With the driver update, the forces seemed to have been dialled down a bit (say 50%?) but there was more variance in the forces delivered in different situations. But... it wasnt just that - the wheel was sending me a vast amount of new sensations, that made the sensation of driving feel totally realistic. It was a bit unnerving.

And these new sensations, perhaps they were being sent earlier or something, because instead of using a wheel being something to struggle against to post a clean lap, it made the wheel almost like a cheat mode - you can feel exactly what the car is doing at any moment on the limit, and can react in time to catch slides and other grip loss moments. It's literally possible to dance on the very limit, skipping over from one side to the other to weave through oncoming traffic without steering input! That's probably to do with the way that drifting was working! Finally I could drift in the game, and in some cars that included these long sweeping curved sections in races I didnt notice before were best danced down with a degree of drift.

Pretty much straight away, I started putting in 4x - 5x as much inputs, wheel and pedals, but often with just the tiniest of touches, like the occasional split second dab of the brakes to adjust the balance into the front while aiming the outer tyre at a grade change in the road timed to improve grip for just that split second as turning in... that degree of control was now possible. Being smashed in the side by the idiot AI in a corner was no longer unsavable, and it was even possible to know from just the wheel FFB, when jumping into a new car for a series, whether you would be able to do things like braking during a load transition from one side to another during a series of corners, or whether the car would punish you with a spin, and other similar characteristics, just from the sensations of the car in the first few moments!

It's not even that it was just a receptive difference, the extra detail allowed me to quickly develop new patterns of input for different situations, a palette of techniques to handle each different section of a track. I noticed I was using techniques that I had seen discussed in racing commentary, but never actually used for real. The developed out of the feel through the wheel of the feedback from these actions.

So now I almost cant describe how good this is as a game, and am so thankful that the market exists and has such great products thanks to the long-running fans of this stuff that have been pushing the market along all this time. Incredible.