I recently switched to iOS and found alternative for those. For YouTube without ads and background playback you can use Brave. For syncing files between devices AirDrop is easy and doesn't require internet. There is an official Anki app for iOS where AnkiDroid is a third party app. In regards to extensions Safari does support them, along with Orion, but I decided to forgo using them.

>real alternatives browsers

Using webkit as the underlying engine within the different browsers on the platform has provided enough customization. The parts I actually care about in regards to the browser are the parts actually handling the user experience and not the engine itself. Webkit has evolved enough that it is good enough for my needs at least.