I've heard the term "wear bars" for years but was never sure what they were. A little googling helped. In the second photo, in the center of the tire, I'm pretty sure they are the little bridges in the channels. When the tread wears down so it's a smooth "walk" across the channel, it's time to change the tire. That said, I appreciate the opinions offered that with cracking/dryness showing and a hot climate (6 months a year), it would be prudent to change the tire fairly soon, and I shall. Thanks.
"Wear bars" are the "legally allowed" minimum. (In my country, being 1mm. No idea what that is in imperialistic terms). That is where the police can fine you for unroadworthy tyres, or where , if in an accident, the insurance can deny your claim (at least they do in my country....)
I also have the same issue as the OP. My C600 Sport Rear "Pirelli Diablo" is a 4615 tyre, and the front "Pirelli Diablo" is a 1616 tyre. Rear getting on for 7 yrs next year, but LOTS of thread left, and NO dry cracking. (It sleeps in a garage overnight, and winter and summer temps are very mild throughout the year). I thus decided to ride it another year at least.
My previous C600 Sport, I had on a new set of Metzeler FeelFree's, when the bike(scooter) was written off early this year, unfortunately.