Connectives such as *fa* may signal sequence or consequence, while *thumma* introduces a kind of interval. Their specialized meanings help define the relation. Wāw (الواو) ordinarily coordinates without specifying those relations, so its use between complete clauses calls for special rhetorical judgment.
«في حكمه عطف عليها كالمفرد»
In the relevant syntactic respect, the second clause may be conjoined to the first much as one word is conjoined to another.
The governing question is: Is the speaker actually making both clauses partners in the same construction or judgment? If not, adding wāw may create an unintended relation.