Hello Aineias,
Thanks for joining and posting your plea.
I’m not sure how much of this is going to help:- but…
S Line has sports suspension so will have an inherently harsher ride, and will be biased towards road holding rather than comfort. As I see it - and I could be wrong - this package comes with equipment that is attractive/ appeals, but the package has a compromised ride quality if comfort is your bag.
18 inch wheels are ‘mild’, and should give as near to ‘best for specification’ ride as possible, so down sizing to 17 inch may still not show you significant improvement, since you now have a rather generous 50 profile set up. That profile will have to increase if you downsize to 17 inch, and you may find it difficult to find tyres of a bigger profile than 50 anyway.
Were 17 inch wheels a specified option for your vehicle? Handbook should clarify.
Questions re. existing tyres - Are they ‘extra load’. (XL) tyres? If so, the sidewalls can be harder than non-XL.
Are these tyres ‘Run Flat’ by any chance? Again adverse affect on ride quality due to sidewall strength.
Perhaps you could come back to us.
Kind regards,
Gareth.