My buddy Dave did that for a month after one tutorial.

Plastic welding is a lost art and that kid will figure it out when his parts keep showing up wrong or back ordered for weeks. Insurance adjusters push the swap game because its faster for them and the parts vendors love it. Been there with a cracked cover on an old Corolla and the customer was way happier with the weld job than waiting on a box. Keep doing it your way, the hands on guys always end up busier in the long run.

Switching to linen won't fix anything if you keep using the same stitch and tension. Hemp getting blamed here is just wrong, because linen stretches too when it's not waxed enough or when you pull your loops tight as you go. The real issue is that you sewed 80 pages in one go, so the whole block has room to wiggle and each section slowly works the thread loose over time. Try waxing the hemp harder with beeswax and making your stitches snug but not tight, then add a bead or a knot at the head and tail so nothing can slide. Two weeks of daily use on a soft cover journal is also just a lot of wear, and even linen would show some give by then. What kind of cover and board did you use, because that changes things more than the thread does?