Hey nclt22,
We're grateful that you posted here so we can help you figure this out.
While it does look like this feature is supported on macOS (see here: Check spelling and grammar on Mac - Apple Support), it doesn't look like that may be an option on your iPhone 12 mini. With that being the case, it sounds like the best option would be to leave feedback for the iOS development team. This is a way that Apple provides for users to give suggestions for new features or improvements to existing features that will be passed along directly to the team responsible for that system. In this case, your feedback will be provided to the iOS developers and may be incorporated into a future update. You may not receive a direct response but this is definitely going to be the best way to make your voice heard.
You can share your feedback here: Product Feedback - Apple
We hope that this information is helpful.
Have a great day!