You know what would be even more simple than the Shorncliffe line duplication? Adding a DOWN-UP switch between Sandgate and Deagon. Every alternating downward train could use the switch to stop, terminate, and and turn back at Sandgate stations platform 1. This will give Shorncliffe station a 30min frequency, but since Sandgate station is a much better patronised station and Shorncliffe station has a lower surrounding population than all other stations up the line, this would be a viable solution.
Even more frustrating is that there used to be switches between Sandgate and Deagon stations until they removed it in the mid 2010s. But if they have the willpower to remove a set of switches to reduce capacity on the Shorncliffe line, off course they can put in the same effort to add a single switch at that location.