I don't believe there is any need to remove the foam. I had that foam in fibreglass kayaks for decades with no problems.

Removing the foam won't do anything at all to change the tracking. The reason it is hard to keep straight is that it looks a bit like an old slalom kayak. They are not designed to go straight. They are designed to turn.

You have two options. Persist with learning to control it. With the right technique, it is possible to paddle them in a straight line.

The other option is to fit a small skeg near the stern.