Caramels turn out too sweet

As you mentioned it is all about how the crystals form.

Some of the factors off the top of my head:

• How saturated is the solution - The more sugar packed into the syrup the more easily it will crystallize.
• How quickly it cools - The slower the bigger the crystals
• Interference - Do you have a starch or other sugar molecules gumming up the works?

Obviously you already knew this from your question.

Not using other sugars will help. You might also try adding more sugar to the recipe at the beginning to fully saturate your solution. The fact that you add sugar at the end and it still dissolves in makes me think your solution could take a bit more. Letting the solution cool slowly with your seed sugar at the end should help.

You would think it would be easier to get it wrong. Perhaps you simply have too much experience doing it right.

Ask an amateur to try out your recipe and see if they fail properly.

The higher the temperature, the harder the caramel. That is basically the whole story. So I think what is happening to you is that the caramel in the middle is still going up in temperature due to residual heat, while the stuff at the sides cools down quickly because it can vent heat through the pan to the outside world. Have you checked the calibration on your thermometer? In any case, I think the things you can try are (1) cook to a slightly lower final temperature (back down a couple degrees at a time) and (2) set the pan over a tray of ice to cool it more evenly. Also, be sure to do a really good job of that final stir before pouring into the tray.