https://pastebin.com/uZYmRxb7
If you have questions, ask and I'll answer to the best of my ability.

With CustomCubic, you are not supposed to enable modded generation. It's never going to work correctly.

With Default+CubicChunks/VanillCubic, some mods are still going to crash. I know why and it is fixable.

As for slow generation - it's also a known issue, a combination of generating more stuff, slower generation and slower lighting.

Minecraft ran out of memory while it was generating the world. I had 6Gb dedicated to Minecraft.

Possible memory leak during world generation?

Can you show the log? fml-client-latest.log and latest.log and if possible, launcher console output with logging level DEBUG

correction again. 5Gb, setting to 7 and trying again.

Another correction, greenhouse module of Forestry still catching on something with the large mod pack and the normal cubic generation.

1. Try with CubicChunks-1.12.2-0.0.821.0-SNAPSHOT-all.jar. The "all" gives the whole thing.
2. Also try with malisiscore-1.12.2-6.3.2.jar.
3. Not all the mods you are trying to play CC with have been tested and shown to work with it. It is better to start with a CC-only pack and add the mods one at a time unless you are sure they work (Chisel, etc.).

Trying now, but I would like to have it on record that none of these mods seem to immediately stop working when Cubic Chunks is present. World gen on the other hand is &”&”$$@ impossible with how long it’s taking. Gave up after an hour and it not even leaving 0%.

I will also heavily suggest compatibility testing with forestry. Because world generation always locks up or stalls when it gets to forestry.
Oh, since I'm running such a large modpack, you'll probably be hearing a lot from me until I able to generate a cubic chunk based world.
One more question. How many different options are there in the generation list will result in a cubic chunk based world when the mod is in?

Correction, Forestry seems to stall on custom, not normal generation. This note was taken with only forestry, malisiscore, and cubic chunks installed.

https://gist.github.com/border999/0a37371643c82b9da7113e5e077a1ca6

will bring up debug and other info as soon as possible.

Quick question that may speed that up. Where would that info be logged, after another instance of Minecraft is launched after it.

Side note: your java 8 is really outdated.
You version: 1.8.0_25
Latest Java 8 Version: 1.8.0_162

Just using the version of java that comes with minecraft now, but I'll upgrade now.

I looked at the crash log again, this looks like you are trying to use the jar without -all, which doesn't contain the required libraries.