best hardwood for first 3d milling attempt and end grain vs side grain

[EddyCurrent]

When buying wood you need to buy the nearest thickness board to your final requirements. Cutting thicker boards into thinner ones generally always results in warping because thick wood has a higher moisture content in the middle. Sometimes if you can cut it, lay it flat, and put a heavy weight on top, it will dry out flat over a period of time, but you'd need some packers under the bottom and under the weight to allow air to circulate all around the wood.