There’s nothing more satisfying than a full and rich sonic landscape where the elements are so detailed. By contrast, there’s nothing as disappointing as a thin and hollow representation of an otherwise great record.
Thinness has a number of causes and therefore a number of different solutions. First, focus on balance. Thinness is the opposite of muddiness. Wherein a muddy mix there is simply too much overlapping sound. While a thin mix is one where the sonics are under-represented. Second, contour your lower mids. Most of our fundamental tones, first overtones, and room tones hang out in this range.
Read more about this here: https://theproaudiofiles.com/thin-mixes/