Changing your run flats to conventional tyres should provide a noticeable improvement in ride quality as the sidewalls will not be as stiff, while changing to 18-inch wheels with a larger sidewall would improve this still further, although it would be important to ensure that the wheels clear the brake discs and callipers. As for remaining compliant with your insurance policy, this is something of a grey area as different insurance companies may have a different opinion so we would advise confirming with your provider, but as a general rule if you choose wheels and tyres that were available as original specification equipment when the vehicle was new then you are minimising the risk of an issue.