What is the current tire size?

What's the width?

In that case the closest conversion to 19' will be 225/45/19 as mentioned above. However if TS already feels 225/50/18 is a bit bumpy then forget about it because 225/45 will noticeably be more bumpy!

Yup, 225/50 is actually still pretty thick profile with 11.2cm sidewall height, so it is still relatively pretty comfortable, not to mention you're using Turanza which is already supposed to be comfort level tire model. The fact that even with such setup you still feel it's rather bumpy means you absolutely would not be able to tolerate anything remotely low profile tires at all! If you upsize to 19'using 225/45/19 as the closest conversion size, then you'd only have 10.2 cm sidewall, so loosing that 1cm will probably brake your back already! So yeah, just forget it and can close this thread.

I think I realize now why you feel it's still bumpy. It's because they're run flat tires!

Yes RFT will have stiffer sidewall to enable supporting the load even without air.

Turanza is actually Bridgestone tire model. ☺️ It's a good mid range category tire, similar like Michelin Primacy.

You may need to carry spare tire though if you use normal type tire.

I'd suggest to at least carry an electric pump. That would be enough to cover most puncture situations so you can reach the nearest tire shop. I have 1 in each of car.

Yes of course.