Shell strength isn't usually the problem. Chick strength is. And that's a function of enough air to breathe and correct temps before and at hatch.
You never get to tell what the "strength/thickness" of an egg is if it hatches on it's own. Guineas throw some of the hardest eggs out there and those suckers pop out like popcorn.
It's about strength of the chick, not the shell. That egg tooth can whomp anything.
You're seeing a weakness problem. Small air cells are showing a chick not the right size, not breathing properly, all kinds of small things that are hard to diagnose are shown in those things.
So you're still really looking at the constants you can effect. Humidity - try nudging it down. Temps sound okay but you could nudge it a bit up for a hatch to see.
Ventilation - adjust it toward more, even if just at hatch time.
I don't even mess with my humidity here until hatch, then get it to 60 and forget it. The hatch raises it.
The only time I fiddle is with shipped and probably heated eggs, where I note a too steep water loss over the course of incubation, when I'm candling. Then I bump it up during incubation and at hatch.
Have a look at the air cell charts in the read me section, and the ventilation discussion. Try to get your air cells right with the humidity you're using and work on your air exchange during hatch.
Likely those two things will get you better hatches.
It's a matter of practice and feel for it that you do get over time. I'm glad you saved some of them. A lot of people fair far worse there first few times out.
I've seen broodies ruin whole hatches a number of times. In truth, they've really no better record than someone learning incubation, because each hen must learn and bad things happen to hens too.
Bad broodies, hurt broodies, eaten broodies, snakes, and found nests are why I got an incubator. I used to do it ONLY with broodies, then got frustrated. Now I've always got an incubator plugged in when I have a broody - so if mistakes are made I might be able to save some.
Sometimes nature sucks as much as artificial incubation.