I’m aware that some Chassanim (grooms) don’t wear any decorative items, such as their golden watch gifted, when they enter their Chupa ceremony. Is there a source to that?

Indeed this minhag is mentioned in seforim dealing with the traditions of weddings and are usually kept by individuals with a Chassidic background. Avodas Yisroel and Shulchan Hoezer 2: n. 136 quoting Birchas Aharon mention that it should be avoided to comply with the spirit of also remembering the day of demise at that so crucial and important occasion (See Hilchos Nesuim, Nitei Gavriel – Nisuin 15: 5). Horav Shlomo Miller’s Shlit’a opinion is that many don’t have that minhag, and one should comply with the traditions of his family and community.