Hello S3P44,
It sounds like you are concerned with the way your keyboard looks when it's rotated in landscape.
Reachability is a feature for Portrait orientation, Adjust touch settings on iPhone. As the layout looks as expected for the Messages app when rotated in landscape, is your concern with the size or the boldface of the text? If so, you can adjust the size and boldface in Settings > Accessibility > Display & Text Size, Adjust the display and text size on iPhone.
If the size of your text works for you, as this is what landscape is supposed to look like for this app, you can always provide your feedback about this app or feature to Apple here: Product Feedback.
Hope this information helps.
Have a good day.
