There are wear bar indicators in the groove of the tread that you should be looking for. Little triangle marks on the sidewall right next to the rim show where those should be across the tire. When the tire tread is even with the wear bar in the middle (or just off the middle if you do a lot of curvy roads and wear out the sides more than the middle) then it is time to replace. I'd say you are getting a lot of miles out of your tires in spite of lower air pressure. Usually the psi recommended in the owners manual will wear the tires just a bit more than a few psi higher will. At least that's been my experience in the last 20 years of owning a bike long enough to need the tires replaced.
I've changed both my tires on mine.
I changed the front tire at 6670 miles in the middle of September 2022. I could have gone another 1000 miles on it, maybe. But I was going on a 1350 mile trip a few weeks after I changed it and didn't want to chance the tire wearing out while on the road. I don't have a photo that clearly shows the wear before I replaced that one. But there was just barely an edge between the top of the tread and the wear bars... just enough to catch my finger nail when I ran it across.
I changed the rear tire at 8303 miles at the end of November. I've attached a link to a photo that shows the wear while I was removing the tire from the rim. You cannot really see the wear bars in the first photo here. I used the large C clamp to break the bead on the tire. Worked out great doing that. The strip of wood was to protect the rim from damage and the screw part of the clamp was pushing on the bead of the tire just off the rim on the other side.
https://photos.app.goo.gl/xV36d5vFdKr1YPPt8And in this photo you can see the other side of the tire. There are a few of the wear bars visible but those are off the center where there is not as much wear. Most of the wear bars in or near the center were level or a little worse with the top of the tread. At the edge of the center where there is more wear you can see one of the wear bars that was not level... yet but other wear bars that close to center were definitely smooth across the groove with the tread.
https://photos.app.goo.gl/ZshfBBrSXhziSmwD7Usually the front tire doesn't flatten out unless you never ever turn or take a curve. But the rear tire will flatten out at least somewhat. Much more than the front anyway. Usually the softer sides of the front tire tread just off center will show more wear due to taking curves or turns and that results in a slightly goofy wear pattern that looks like the center is raised unevenly. This is all normal.
I expect between 6000-9000 miles per tire for any super scooter. But YMMV. Different road surfaces and riding conditions as well as other factors as riding style or aggressiveness and even weather will effect how many miles you'll get out any bike tire. Sometimes the difference can be extreme. I know some riders of the Honda ST, like my ST1100 or the ST1300, who would easily get 9000 miles on a tire... and others of that same bike would only get 3000 miles. But their riding environment and riding styles were drastically different. I would usually get around 6000-7000 miles on tires on my Burgman 650, front or rear.
