Curves. Sample a curve. Round the result and associate it with rarity (enumerator, perhaps?)
Also, this script will not work the way you think due to pure node evaluation. Every time we Branch, a brand new random value is generated. If you want to continue with what you have, ensure to store the generated value before cascading into branches. Why continue with this? Because if that’s all you need and it is working the way you want, there’s no reason to change it.
Opt for curves if you believe the complexity of the generator can get out of hand. They often do. 