Does Rambam ever mention (or reference) the principle that it is preferable to perform every mitzva oneself rather than through an agent? (See e.g. Kiddushin 41a).
The Rambam mentions the concept in at least two additional places, besides Kiddushin.
Hilchos Shabbos 30:6:
חייב לעשות דברים שהן לצורך השבת בגופו, שזה הוא כבודו
[A person] is obligated to do things for Shabbos himself, as this is its honor.
And in Hilchos Sefer Torah 7:1:
ואם כתבו בידו, הרי הוא כאילו קיבלו מסיניי; ואם אינו יודע לכתוב, אחרים כותבין לו
And if he writes [a Sefer Torah] with his own hand, it is as if he received it from Sinai. And if he doesn't know how to write it, others should write it for him.
The Shulchan Aruch HaRav (K.A. O.C. 250 2) explains, that vis-a-vis Shabbos it is a requirement, but when it comes to other Mitzvos it is a hiddur.
The Rambam brings the Din in Kiddushin in Hilchos Ishus 3:19 (3:25 in Mechon Mamrei) and the underlying principle of not degrading Mitzvos (which the Shulchan Aruch HaRav says is the underlying concern in the matter) in Hilchos Shechita Chapter 14 at the end.