You are conflating a browser (Safari) with a web rendering engine (WebKit). You can not use other web rendering engines on iOS, but you are free to use any of t

You are conflating a browser (Safari) with a web rendering engine (WebKit). You can not use other web rendering engines on iOS, but you are free to use any of the 100+ different browsers that exist on iOS. Some of them are privacy respecting, some are not, some of them are ugly, some are beautiful, some of them are free, some of them are paid.