I had started a "builds" world (default world settings) in 1.12.2 some time ago and am adding new builds weekly, hence I decided to make the map like a big city, divided in areas by architectural style.
The Problem
The problem is terrain. I've tried to flatten it with WorldEdit (I have chosen my base level to be 63), but it leaves awful walls at the edges: 
And WorldEdit's //smooth does a terrible job covering them (as seen bellow).
(thank you WE for the stairs)
What I've tried
1. World type: customised
I have tried playing around with the preset settings using this online tool (minecraft.tools) and most specifically with the "micro-relief" setting (I used low values).
This did a great job in matters of flatness, but did some huge malformations with mountains (without touching anything else than micro-relief):
These caves beneath are gigantic (600+ blocks), super common and sometimes even stacked vertically (as the ones in the upper part of the picture). Extremely steep cliffs are also not a rare sight. (On the bottom right you can see a piece of nice and flat plains).
2. World type: super flat (preset with decorations – ores, trees, etc.)
This is now way too flat. No rivers, hills, no regular mountains. It doesn't look natural.
What would be the perfect result?
Mountains and rivers (as a background/panorama) surrounding some big flat areas (plains, forests, etc.) with minimum elevation difference.
If possible, I'm looking for some advice or tips on how to generate a world that is a combination of the two images below:
Default World (as a background to the scenery)
World with low values of "micro-relief" (land to build big mansions, houses, and cities)
Best Answer
If you are not adverse to using tools to edit the world outside of the game, WorldPainter may help you solve what you are after.
It is a self described map generator which "allows you to "paint" landscapes using similar tools as a regular paint program." which if you have not started building yet could help you flatten the landscape with what is essentially 3D painting tools for Minecraft.
I'm not currently in a location where I can test this program out myself, but it is worth a try. It's free, Open-Source and uses GPL. I've seen maps created with WorldPainter before, and if it allows you to create landscapes like this:
Then flattening terrain should be a breeze. As the saying goes, if you want something done right, do it yourself (With the best tools you can get)