Simple solution: NBTExplorer
Our problem is: Too many armorstands causing the game to overload and none of the three listed options working. Thus, removing these entities would fix our problem.
After installing the program, open it and select your world save folder. The file/folder we are looking for is "region"; it contains all chunks and a lot of information stored in it
Now, all you need to do is know the chunk coordinates (they are different from World coordinates) and mine happend to be in spawn chunks."Chunk [0, 0] in world at (0, 0)"
In this same picture we can see the "Entities: 21845". They are the bad boys!
Upon opening it, we can see all of the entities!
And clicking in one of these "folders", they show us valueable informations, such as the type, tag and even their custom names!
Now, there is two things we gotta do to delete all of them:
First, copy the precious ones to another place, it can be anywhere.
it's important to copy them since you cannot cut more then on per time. To select them all, just press shift and left click... yeah, it's the only way... Luckly I had to copy 10 only.
After that, delete the 21k entries 'entities' folder and copy your backup/precious ones back there. Remember to also delete the old backup folder.
Lastly, save it. It will not auto save if you close it.
If by any means your tab looks like this:(Pic bellow)
Just re-open it. It's normal to crash if there are that many entities.