Rav Schwab on Prayer discusses the idea that the large ד in echod is meant to remind us not to read the words as acher, as in the pasuk “For you shall not prostrate yourselves to an alien god (Shemos 34:14).
Rav Schwab then quotes Rav Hirsch from Devarim 6:4: “The ר of the polytheistic thought is accommodatingly rounded, while the ד of the Jewish truth is sharply angular. With the loss of a little sharpness, the אחד becomes אחר . The meaning is quite clear: Hashem Echod is a sharply defined concept, it is this way, and there are no other possibilites.
It’s when I try to cut corners that I see myself go off target in my Avodas Hashem. Halacha is th “sharply angular” clearly defined parameters that we need to work in between.