I ended up overcooking my quince jam. It is really overcooked. It's not burned or anything but the quince pieces have harden up due to loss of water. Can I use it for something else? I don't wanna through it as it would be 6Kg of waste.
You might consider making your own membrillo.
Quince cheese, also known as dulce de membrillo, is a sweet, thick, jelly made of the pulp of the quince fruit. Quince cheese is a common confection in several countries, where it goes by various names, such as [. . .]
It's a really great option and pairs beautifully with cheese and crackers. I've never made it before but there are some guides on the web and, considering it's essentially a very dense jelly, it may be ok with your dense quince fruits.