My sugar topping became very runny after cooling and sitting a couple hours.The peaches weren't super juicy, just nicely ripe and slightly firm. I...

When I make peach pies or tarts, I always mix the sugar with the peaches and let them macerate at least an hour. Then I drain off the juice and reduce it in a saucepan until it's a bit thick. Let it cool a bit, then return to the peach mixture. You'll still get more juice from the peaches in the cooking process, but less than you would have otherwise.

Since I love this recipe and will be making it a ton until the peaches disappear, I have the chance to experiment with all these great ideas. Thanks so much.

You could also try stirring a bit of sugar into the sliced peaches and letting them sit for a few minutes to draw some of the juice out, and leave the juice behind when you place the slices on the tart.

Yes, that sounds like a good idea - or you could add a drop of cornstarch also. I feel like it tends to work better than flour.

I think your idea to add a bit more flour is a good one. I'd start with 1 tablespoon.