I hike down there a fair bit and actually just got back from one. We did Tidal river, Oberon bay, Halfway Hut, OLD telegraph walking track to the lighthouse, then back up to Waterloo bay, across to the junction and back up Oberon to Tidal.
Telegraph track is the most batshit boring walk on the Prom. 12-odd kilometers of marching through boring scrub down a gravel driveway. But easy. If you had an extra day, I'd advise walking out and back though Tidal/Oberon. But you dont. Bummer. At least take the old walking track to Roaring meg that follows the actual telegraph line over the high plains. It's a small sign turning right off the road, once you get over the hill south of Halfway Hut.
The lighthouse is not just a quick trip from roaring meg. The shorter walking track is still a solid 5k hike, with a bit of up and down (unless you just keep slogging down telegraph road, which actually adds a km) and the monster hill to get to the lighthouse proper. Get to roaring meg by mid morning if at all possible. Lighthouse and back from roaring meg is a full afternoon with April sun, assuming you don't dally at the lighthouse...
...which would be a total waste. It's arguably the best spot on the Prom. The views are amazing, you can swim at the old boat loading cove, the wombats are basically tame and will eat under your table as you sit there, and the lighthouse tour is legit interesting. I assume you're not allowed to stay the night there in the cabins? If not, you're gonna want to allow at least an hour or two there to really enjoy it.
I assume by "Sandy Point" you mean South point (Sandy point is miiiiles further north) If so, just be aware, you can't camp at south point, according to the rangers. Of course, there are lots of places you "can't camp" at the prom, but people still do. Just be aware, not an official (legal) camping spot. Also - in my opinion, the views are better at the lghthouse.
All in all, I think you've chosen the best route for a one-nighter. Get going early, get to roaring meg before lunch and save lots of time for the lighthouse