When copying from 𝓞 into 𝓟, Oliver Cowdery initially wrote “so many things”, then immediately corrected the phrase by supralinearly adding marvelous (there is no change in the level of ink flow). Undoubtedly the original manuscript (which is not extant here) read “so many marvelous things”. Otherwise, there would have been no motivation for Oliver to have made this correction since “so many things” would have worked. Elsewhere, the original text has 18 occurrences of so many modifying a following noun phrase, of which two others involve an adjective (“so many precious things” in 2 Nephi 5:16 and “so many brave men” in 3 Nephi 3:3). All the evidence argues that in 1 Nephi 1:18 “so many marvelous things” was the reading of the original text.
Summary: Maintain “so many marvelous things” in 1 Nephi 1:18, undoubtedly the original reading even though 𝓞 is not extant.