How to make a meringue cake layer that is not cracked?

I am not entirely sure that what you intend is doable. A pure French meringue, when baked enough to get hard, will generally go brown on the outside. Also, I don't think the idea is very functional - first, most fillings and icings will likely wet your layers through, and second, if you can keep them dry, eating the cake will not be especially pleasant, getting worse with every layer you add.

You may instead look into existing recipes for cake layers designed around a meringue. These are usually dacquoise cakes, but I recently also found out that there is a popular cake in Serbia made with what was called orehovka in eastern bloc countries, essentially a merengue-and-walnut cookie, which gets blown up to cake layer size for that recipe.

If you absolutely insist on using pure meringue, you will get better behavior from Italian meringue. Be very precise in measuring the temperature of the eggs and the syrup correctly, in getting it to the right stage without overwhipping, pop it in the oven (your 90 to 120 C range sounds right) and keep your fingers crossed.

In case you continue trying the pure-meringue-route, there are two more things to try. One, you can also add a vessel full of water on the bottom of the oven to reduce the probability of cracking. Two, if you use a silicone or fiber mat instead of the baking paper, you will have more insulation from below, and also less risk for cracking after the baking, during peeling off the mat.