I am relatively new to martial arts with only 18 years studying kobayashi shorin-ryu. So my opinion would of course be limited by my knowledge and subject to change as I progess in training.
But Naihanchi Shodan would be my choice. The bunkai is in a range most self defense situations will end up. The pull/strike, elbow, and kick/sweep/stomping techniques can be very effective and powerful for someone of limited training and small stature. The blocking and hand sweeps mesh pretty well with a flinch response. And the angle of attack on most techniques is pretty forgiving.
The conditioning training is pretty good. And it does teach great power generation. The angle and power principles in my experience are easier to get a grip on then say gyaku tsuki.
Probably the biggest reason for choosing Naihanchi Shodan is the grappling and ground fighting bunkai. Many MMA fans or BJJ students will recognise the guard, half guard, sweeps, submissions, elbow strikes, and mount escape.
With only 3 months to train its a big ask. But I do think there is a breadth and simplicity of technique that could certainly improve one's self defense chances.