Of the few things I am able to do, I was actually trained as a tire vulcanizer and I spent 7 years as a commercial tire salesman.
There is a difference between thread rubber and sidewall rubber.
When a tire is made the body of the tire looks like a barrel then the thread rubber is rolled on.
The thread rubber is made for abrasion while the sidewall rubber is made for a certain amount of flex and its ability to resist weather cracking.
Different companies especially over-seas companies maybe using a cheaper type of rubber for the sidewall. That may explain why your sidewalls are looking rougher on some tires more than others.
The old American brand made tires were the best. BFG tires were excellent tires especially the Rice and Cane tires itís a shame they are out of business.
The tire that has a split in the thread can be repaired using a product thatís made by rema called A&B compound. It is a cold vulcanizing compound.