How to fix bathtub blemishes

Depending on the manufacturer, the stem should be a free replacement. It's fairly trivial to replace.

1. Turn off water. (IMPORTANT!)
2. Remove the large hex fitting on the face of the controller
3. With vise grips, pull and wiggle the stem out.

If it's a common manufacturer, you should be able to exchange the stem at your local big box store or plumbing supplier.

Reinstalling is simply the reverse.

While picking up the stem, get an appropriate bolt to fit, and you're good to go.

Caveat: Figure out the manufacturer and call first.

I'm guessing these shattered because of shock or perhaps from being fully extended more than normal since the front wheel takes a lot of the weight when drawer is fully extended. The rail is acting as a lever where 10kg in the front of the drawer could easily be the equivalent of 100kg on that wheel when the drawer is fully open. The other thoughts are if someone where to lean on the drawer or if the cabinet was being moved and dropped.

I've seen replacement wheels, but they tend to be for specific rails/sliders, and I doubt you'd get a good result without a perfect match. For replacing the entire slider, I'd look for a replacement with a high weight limit. I've seen sliders in the US that are designed for 50kg, which I'm guessing would more than handle your load.