So, your goal is to strengthen the intrinsics. What exercise is best? Probably the most specific one, right? Well....maybe. These 4 exercises seem to all hit them.
This study looked at the muscle activation of the abductor hallucis, flexor digitorum brevis, abductor digiti minimi, quadratus plantae, flexor digiti minimi, adductor hallucis oblique, flexor hallucis brevis, and interossei and lumbricals with the short foot, toe spreading, big toe extension and lesser toes extension exercises with T2 weighted MRI post exercises (perhaps not the best way to look at it) and shows they all work to varying degrees.
"All muscles showed increased activation after all exercises. The mean percentage increase in activation ranged from 16.7% to 34.9% for the short-foot exercise, 17.3% to 35.2% for toes spread out, 13.1% to 18.1% for first-toe extension, and 8.9% to 22.5% for second- to fifth-toes extension."
Gooding TM, Feger MA, Hart JM, Hertel J. Intrinsic Foot Muscle Activation During Specific Exercises: A T2 Time Magnetic Resonance Imaging Study. Journal of Athletic Training. 2016;51(8):644-650. doi:10.4085/1062-6050-51.10.07.
link to full text: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5094843/