Is my plan for a new bathroom lighting and fan circuit code-compliant?
Since you already have a 20A small appliance branch circuit feeding receptacles in the bathroom in question, your plan is fine.  NEC 210.23(C)(3) requires that at least one of the bathroom receptacle outlets is on a 20A bathroom SABC, but still allows additional receptacles to be on a 15A circuit.


  (3) Bathroom Branch Circuits. In addition to the number
  of branch circuits required by other parts of this section, at
  least one 120-volt, 20-ampere branch circuit shall be provided to supply a bathroom receptacle outlet(s). Such circuits shall have no other outlets.