The source and direction of pain, lower back, buttock, thigh then lower leg all points toward sciatic nerve. Possible spinal stenosis piching a nerve.
That would also explain why it happens weight bearing as opposed to non weight bearing on a bike.
I would try some specific exercises to release that first. possible a series of massage,/manipulation (not the happy ending feel good variety) and mild pain relief medication before exploring further.
Read this:
https://www.healthline.com/health/back-pain/sciatic-stretches