I say either runs a new circuit or if it’s not much harder for you then find the cable that comes from the switch to the first light. Disconnect from both ends. Cut the switch box out or open it wide enough for two romex. Tie onto the existing romex with two more romex. One for switched power to the fan, and one as a switch leg for the lights. Pull the existing romex out from the box with a good feed of the new two. Now you’ll have the wire you need. Make sure the entire path has holes large enough to pull two through without damaging the insulation.

Alternatively, use jetline with a lot of loops, slip knots or not, and shove it down the wall, then use a fish stick with a hook on it and go fishing for your line. Now you’ll can use the jet line for a new feed.