>so I became an employee, but that turned out to be a massive step back in terms of income. Despite the fact that I worked closely with lots of stakeholders and solved complex problems for them, their internal rules didn't allow them to pay me as more than a code monkey.

Surely there was a negotiation step before signing contracts? What happened there? What was the blocker that did not "allow" them to change their own internal rules that they themselves control? Surely there is a way to do that.

>I left, they ruined the application (it's apparently slow as molasses now), and now I'm about to go back

Then state what you want before going back, if it's important for them they will find a way. Don't accept these kinds of zero effort "oh our policy doesn't allow us to pay you more" explanations.