What tyres have you got at the moment?
I've got the standard 215/45R17.
It shouldn't be that bad on 17" wheels.. I think it is likely something is worn out in your suspension. If you want a 2nd opinion I am only up the road from you in Cumbria, happy to give it a once over. You'd be amazed what an MOT test will miss, bottom arm ball joints in particular.
I had the suspension checked over as it was knocking like crazy, had new upper wishbones fitted, but it probably needs lower bushes and ball joints etc just to get back to something like a level playing field so I can address it properly. Thats a kind offer, I'm over near Barnoldswick so not a million miles away. I'd like an enthusiast's opinion as your average garage mechanic's eyes flicker with pound signs when you bring in a mid-2000s Alfa with chocolate wishbones...
Whereabouts in Lancashire are you? I can probably find a set of 156/147 16" wheels for you to try to see what you think before you go to the expense of replacement wheels - I'm near Burnley. I actually find with the state of the roads I do my local running around more in the CRV as its 70 profile tyres make for a better ride.........
I'm near Barnoldswick, thats a very kind offer - I'll probably take you up on that! The roads are just awful, most people have SUVs, I zig-zag everywhere I go, and make my passengers very ill!
You need to jack the car up and get right under it. Check every part of the heat shield. Check all exhaust brackets. Mod the parcel shelf so it's stops rattling and the tail gate. You will probably find that sorts most of the harshness out and it's all sound.
Now start looking at changing the dampers. B4 or TRW if on a budget, maybe some Eibachs springs, but of course check for play in the upper and lower wishbones.
Rattles don't bother me that much, this is the most modern car I've had after years of 80s cars and older classiscs, its the forces that I feel coming through the front end of the car that really bother me, which means that the shock absorption is probably the culprit like you said, it feels like the suspension is always at its limit. While they're in there I can look at other bushings too.
I might try some 16s as a quick fix, if I can borrow some that will be even better.