Most of the problem with economy isn't how oppressive or long lasting, it's how simple it is. Or how easy it could be for someone to get unlucky early on and have a vastly harder time paying it off (especially while they're learning the game)
Something very very basic to increase the complexity, which i'm not advocating for, merely discussing, would be to 'force' the player into a negative spin to begin with (debt grows faster than income), forced to save up and buy some sort of other equipment or long lasting debuff (reputation as a criminal) and only at this point their income outstrips the rate of growth
Hell, if the project goes a long way, it might even be valid to have a small basic explanation of how to pay off debts - saving up to get an income is good, but the more you pay off, the more take home income to pay it off you have also
I'm still balancing, but Alpha 3 is ready to post, I'm just tweaking the interest rate to force the player into buying upgrades to make more money to pay off the debt. I have decreased the rate of failure just a bit so now it's more about grinding to pay off chunks of debt rather than worrying about the repo man.
This was the easiest way I could implement your suggestion, but I still need to balance the economy around it:
