How to play customized world on Minecraft 1.13 or later
1)Create customized world as you like on 1.12.2
2)Build a 1.12.2 Spigot Server and copy the world data
3)Install World Border plugin and issue command "/wb fill" to generate the customized terrain forcibly.
4)After generation, copy the world data to the 1.13 environment.
That's all what you do. Please note it takes several hours.
Now we can enjoy our customized worlds again!
I will never waste your effort of tuning complicated parameters.