The lexical approach may assist. The lexical hypothesis states that, ‘All aspects of human personality, which are or have been of importance, interest or utility, have already become recorded in the substance of language’(Cattell, 1943) and, ‘When an idea is important, people are likely to have a word for it … the more important something is, … the more words there are likely to be’(Miller, 1996). The atlas of personality, emotion and behaviour uses a lexical approach to visualise words using the axes of affiliation and dominance (Mobbs, 2020).
narcisism: conceit*, egocentricity, egocentrism, egoism*, egomania, egotism, narcism, navel-gazing, pride, self-absorption, self-centeredness, self-concern, self-interest, self-involvement, self-preoccupation, self-regard, selfishness, selfness, solipsism, vanity.
megalomania: conceit*, conceitedness, egotism*, grandioseness, grandiosity, obsessionalism, paranoia, self-importance.
The visualisation according to the atlas approach is shown below:
In answer to your question, there appears to be very little difference between megalomania and narcissism from a lexical perspective.
Cattell RB. The description of personality: basic traits resolved into clusters. J Abnorm Soc Psychol. 1943; 38: 476–506. https://doi.org/10.1037/h0054116
Miller GA. The science of words. New York: Scientific American Library; 1996.
Mobbs AED (2020) An atlas of personality, emotion and behaviour. PLoS ONE 15(1): e0227877. https://doi.org/10.1371/journal.pone.0227877
Declared interest: I am the author of the atlas paper.