The Mini is not any sturdier, the size seems not to matter that much since they are designed very differently. JerryRigEverything tested the bigger M1/M2 iPad Pro and it actually ended up being one of the sturdiest tablets. I do not see a test for the smaller iPad Pro so who knows if that is as sturdy...
In the end all tablets are relatively thin and as sturdy as the big iPad Pro is, I still wouldn't put it in a backpack's laptop compartment where it will be pressed in between the contents of the backpack and your back. Instead, it should go into a backpack's tablet compartment in the front (furthest away from your back), many backpacks have that and it often fits tablets up to 11" perfectly.
That way no stress is exerted on the iPad no matter how heavy the backpack may be.
There have always been reports of iPads bending under normal use without excessive force and I think that might be what's happening there, people carry it in the laptop compartment pressed against their backs. Our backs curve and if the iPad gets pressed against that over months and years of daily commuting it's not too different from accidentally sitting down on it.
I understand that these are everyday devices that get carried around a lot and need to be sturdy enough to be fit for that purpose. And that might indeed mean that the 10th gen iPad is so easy to break that it's not quite up the task. On the other hand it should be clear that these thin tablets will never be as sturdy as a laptop and can't be transported the same way.
Also is it true that with AppleCare you get unlimited repairs now with copay?
Yes. You can check the prices for exchanging an accidentally damaged iPad here:
https://support.apple.com/ipad/repair It's essentially $49 per replacement. Unlimited does not really matter, Apple just introduced it because it sounds better and makes you feel better about spending the extra money for AC+ in the first place, but most iPads never see any damage, and even if they do it's even less likely to happen multiple times in a row.
If you do end up purchasing AC+ you might as well buy whatever iPad you want without regards to how sturdy it is. If carried properly they won't bend.
I left my 12.9" iPad at the back of a backpack, in its designated cushioned place.
Cushioned just prevents scratches, unless the cushioned place was a metal box I don't see how that would help? In the back where laptops go? That is not the right place for it. The iPad should sit in the front in a tablet pouch where it sits freely without pressure from either side. You might need to replace the backpack if it does not have that or if it's badly designed with the tablet pouch in a bad spot.
Good luck.
