Time to replace tires, tread not issue, age is
This winter I was planning to put two new tires on front and rotate, front to inside rear. Took off rear wheels to do brake check and lubrication and found original tire that looked good in place has separation. Thread is 10 compared to new of 11 on my gauge. Now my best move would be to replace all 6 keeping one for spare.
All rears were 1998 and front were 2004, Michelins 8 years versus web forums 5 makes sense.