How can I forward delete on iOS?
Nope. In the stock keyboard provided by the API, a "Forward Delete" (as opposed to a Backward Delete/Backspace), none of the keys, nor combination therein, provide a Delete keystroke to occur. Your only option, as you now see, is to use an external keyboard to accomplish this.

Any other options would just be something else entirely. Like selecting a word, and deleting it, or using replace, etc.