The treads are still visible...but the middle section looks a bit worn to me. I'm not sure if there's any tread wear indicators built into this tyre. Anyone think I should change?
If utterly maximizing your tire lifespan is very important to you, then you can ride until you see casing threads in one spot. I would not suggest that most people do this. Generally, with worn rear tires, when we look at them, they become flat at the top. Riders may feel like they get more punctures on worn tires - I had a couple tires where the tread wear indicators were not worn off, but I felt like I was getting more punctures and I elected to change the tires. You do get more punctures because there is progressively less and less material, so a shard of glass has less distance to travel before hitting the tube. Both of these criteria are admittedly subjective. I don't see a way around it.
For gravel tires, most of which have treads, wearing the tread off the center of the tire is an indicator of worn out tires. Here, the model doesn't appear to have had tread in the center. Even if it did, I agree that it's still usable.
Good enough to ride on.
The surface is still smooth (no cuts or the internals shining through) - I can still see rubber. I personally tend to change mine when they have too many cuts, less material punctures easier and objects may punch through existing holes. That's usually way before they are totally worn down, for example if I have to change a tube at home (losing pressure, valve issues), I can put a fresh tyre on the front and relegate it to the back if still looks good.
There's still some life left in those tires.
You replace tires when you see the cords from below the tread, and if you do, then you're in a hurry, you really need to replace the tire fast. I don't see any cords visible in your photograph of your tire. Road tires do not require a tread pattern, in fact slick tires work the best.
Some road tyres are having two little holes in the running rubber. They serve only one purpose - to show you the actual depth of the rubber one still have in hand. When the hole(s) disappear, your tyre is to thin and the manufacturer suggests you replacing it.
Other option how to check the status is deflating the tyre and squeezing it form the sides. If it is very easy to fold the tyre or when it folds inwards - in other words it the tyre walls support the tyre more than the runing patch - then it is time to change the tyre.
The thin rubber does not affect the riding comfort much. On the dry tarmac missing thread does not harm the handling, it is better, actually. On other surfaces (wet tarmac, sand, mud,...) the thread is critical for the grip.
In this case, the thinner tyre is just more prone to punctures.