I'd reckon that any where like KwikFit etc, that fits tyres to your car, would always advise and fit the "correct" tyre for that car. If they did not they would end up in court when or if things turned to sh**e - ie after an accident when the driver tried to plead ignorance to the error of fitting "other" tyre sizes.
Yes the reason to drop the aspect ratio of the tyre would be to keep the wheel (tyre) rolling radius the same to avoid speedo errors.
To the OP, you seem to want a more "forgiving" ride, to do that as you have worked out, you would need to increase the aspect ratio - but you seem to be trying to do this AND go for a wider tyre - that will lead to speedo errors and change the overall gearing of the car.
I'd reckon that if 205/40 R 17 is the correct tyre for the car then you have no where to go to improve things and keep your insurance happy.