Why would you want to use another software keyboard than iOS built-in one? How about because you wouldn't have to switch keyboards when you switch languages?
For a long time keyboard apps didn't work very well. It was mostly not their fault but due to iOS itself. Now they work a lot better.
One of my favorites is SwiftKey Keyboard. It has text prediction. That means it tries to guess what you are about to write. It also learns your writing style and adapts to it.
You can choose two languages you often alternate between in the apps settings. Then SwiftKey will detect which one of them you use and adjust text prediction to that. I, for instance, write as much in English as in Swedish, my native language, and save a lot of time by not having to switch keyboards manually. This is the main reason for me to use SwiftKey.
Other good features of SwiftKey is that it has several keyboard themes to choose between and SwiftKey Flow, a feature that let's you swipe between letters whole you write. It's said to save time.
The app is free in the app store.