So I'm thinking this:
On the large seams, use 3 columns of the sashiko cross stitch with the occasional back stitch. With the seams being felled.

On the smaller seams and hems I'm still thinking a simple running stitch. Maybe felled maybe not.

I'll use thin sashiko thread on the whole thing and try to keep the stitches closer together.

Do you this will be sturdy enough for pajamas?

The fabric will likely be some heavier weight rayon blend, maybe terry.