I've been making bread by hand for 40 years but have never solved the problem of my loaf top collapsing during baking. I use total 1kg flour: 70% whole meal and 30% strong white and use Tesco's dried yeast sachets. I mix by hand, prove in a warm oven, knock down and do second rise in three tins.
I normally use 1 sachet for 1kg flour which is half what it says on the yeast packet. But I've also tried using 2 sachets. I've tried more water, less water, conventional oven, fan oven, hotter oven, cooler oven. Preheat oven, don't preheat oven. Single rise in the tins. I've tried three different makes of flour. I even tried fresh yeast. Nothing has made any difference! What could I be doing wrong?
Is a sure sign of over proofing. Shorter final rise will fix that.
Even if bake before the dough rises to the top of the tins it collapses.
The hole size seems fine to me. This image is the latest loaf I made using 2x7g sachets of yeast to 1kg flour. This loaf was 1/3 of the batch.
Then the pan needs to be filled more. Make sure you're using the right sized dough for the pan.