Increase underexpansion of rocket engines in vacuum

This is a more in-depth and fleshed-out version of this post.

The problem.

In SimpleRockets 2, vacuum optimized engines' exhausts look something like this:
Low pressure

High pressure

Notice how the exhaust has a clearly defined boundary and appears only mildly under expanded. However, this is not how vacuum optimized engines' exhausts look like in real life.

Examples:

RL-10

Merlin-1D Vacuum

Merlin-1D Vacuum CG to better demonstrate visuals

Raptor CG

J-2

The fix

You can somewhat emulate real vacuum exhausts by setting the exhaust opacity to ~50% (via overriding the color) and setting the exhaust scale to 0.1.

The effects are amazing.
Low pressure

High pressure

However, this makes the exhaust look awful in the atmosphere.

Summary

Rocket engines should have extreme under expansion effects regardless of exhaust pressure in a vacuum. Rocket engine exhaust should also be less opaque in a vacuum.