Hey mate,
We do have primitives in MonoGame.Extended. It was originally ported from the Primitives2D project (with permission of course). I believe we’ve fixed a couple of minor bugs along the way.
I agree with @Trinith, extension methods on the SpriteBatch are convenient because you can just use it whenever you want. Very good for debugging issues. The statically created Texture2D is not ideal and rotated lines are not perfect.
We’re working on MonoGame.Extended 2.0 these days. Improving the primitive drawing code is something I’ve got on the to-do list.
In the meantime, feel free to borrow any of the code you want out of our library.