Frequently used words, idioms and idioms can be quickly entered by registering them in the "(user) dictionary", but the task of entering combinations of character strings and readings is quite difficult with an iPhone that uses a software keyboard. It is penance. Copying and pasting is less efficient than a computer, and you may not want to work unless you have a lot of time.
If you have a Mac at hand, there is a way to register it on the Mac. Prepare a Mac (macOS 10.15 or later recommended) signed in with the same Apple ID as the iPhone, and open the "User Dictionary" tab in the "Keyboard" pane of System Preferences. There, the user dictionary used on the iPhone should be displayed.
Press Command-A on the screen and drag and drop the highlighted part to the desktop. A file called "user dictionary.plist" will be created, so open it with an application such as TextEdit (a text editor that supports UTF-8). It defines a number of dict tags in which strings and readings are arranged in a one-to-one relationship.
I omit the detailed explanation,
Next, let's drag and drop "user dictionary.plist" to the screen of "user dictionary" tab in "keyboard" panel of system environment setting. If the added phrase / reading combination is reflected on the screen, the work is completed, and after a while, it will be reflected in the iPhone's user dictionary via iCloud and will be available in Japanese input.
Easy explanation of operation procedure