At the moment we have no intentions to change the range of stylings. Certain combinations of headings and inline styles like Italic and Bold can indeed be hard to distinguish, but this naturally depends very much on how you use them in combination. We want to prevent adding many more options or turning Agenda into more of a word processor where you can control every aspect of formatting. Instead we would in the future be more looking at allowing different themes, where different headings could feature a different style, which could address your issue by allowing you to pick a style that works better with the combination of headings and styles you use most.
Truth be told, themes are probably a while out still so in the mean time you might want to consider using a different combination of styles and headings, for example by using h4 instead of h3, or use horizontal rules, plain text prefixes like “+++”, indentation, or emoji’s to help visually better delineate different sections in your notes.