You can't replace the system's touch keyboard. Some apps can and do modify the keyboard within those apps. You can do your typing there then cut & paste to the app you want, or maybe the app does what want. To the best of my knowledge this is the best you can do for the touch keyboard. For now at least.
Storyist is the only app I have that has cursor keys. That's probably overkill for what you want.
Most people who want a better keyboard for typing on the iPad (for more than a few sentences) go with an external bluetooth keyboard; like Mtnmedic's suggestion.
There are lots of options, as most bluetooth keyboards will pair and work with the iPad. Apple's keyboard is popular, and Amazon just came out with what looks like a decent keyboard for a little over half the price. It is called the AmazonBasics Bluetooth keyboard but made very much with iPad's in mind. Also in this group are special mobile (folding) keyboards and keyboard cases for the iPad.
Try a few searches in the forum and on Google. You'll find plenty of options to mull over.