I bought a 65K mile '04 last August. The single best thing I did for the ride of the car was get rid of the cheap tires that came on it and replace them with Michelin Primacy tires (still 17"). The second best thing I did was add the Volvo strut tower brace. I have since replaced the entire front suspension (including tie-rods) and the car is much better. But it still shakes worse than our '87 VW Cabriolet (with factory roll bar).
There is only so much you are going to be able to do.
We swapped the 16" wheels and Michelins from our '04 V70 and put the 17" on the wagon. Both vehicles were improved in appearance, I believe and I 'think' the ride of the C70 was improved. Didn't help the cowl shake at low speeds or over rough pavement, but the highway ride was silky smooth except for that 51mph shimmy.
One must assume from your shudder concerns that you own a convertible and not a coupe. I do own a 2001 coupe and have not been visited by the shudder gremlin in 95,000 miles. Still, I have tuned the suspension over time with better than stock shocks, springs and swaybars. Wheels are 17x7.5 and michelin tires.
But my tranny did fail and cost $3600 to replace. So, I guess we all have our problems.
Its a beautiful car.