I was the same Antti, but I have bit the bullet and swapped to a UX. I was only considering Toyota Corolla or CHR (new shape), Mazda CX30 or Honda (civic or ZRV). I ignored bmw and Volvo due to possible reliability issues. In the end my UX came up and I liked the look and the Takumi spec was the same as my IS Premier with the extra modern (sometimes annoying) safety features. I have done a couple of hundred miles in it now and I am completely happy. The Dunlop runflat tyres are no noisier than the Michelins on my IS. The IS300 was the best car I have had and you are right it is a hard decision to change, the lower end Lexus cars (UX, LBX) are not built quite the same. My old car has been sold within two days of trading it in, somebody has got a really well maintained car for not a lot of money nowadays. Regarding the warranty my last service was before it’s 10th anniversary and 100k miles so actually got 11years.
Good look in your search or just keep your IS and enjoy it.