You would need a collection of csv files (for planets, fleets, etc), so that adds some complexity.

JSON is a key-value format, it essentially allows 'objects' to have keys with values that can be primitives, more objects, or lists.

Any csv format, say headers a,b,c and rows 1,2,3 and 4,5,6 can be trivially converted into a (more verbose) json file [{"a": 1, "b": 2, "c":3}, {"a":4, "b":5, "c":6}]. The verbosity is of course a drawback, but it makes it easier to read/write as a human and gives more flexibility for e.g. adding keys later, ommitting certain keys for certain rows, adding embedded 'rows' (for example for ships in a fleet) etc.