Skrooge is KDE-based, but will work on any system. It isn't really designed for business and freelancing but for personal accounting, but that said, it's great and might be all you need, and is worth considering. I use it for my own self-employed business accounting as well as personal stuff.
Tangential: the best frugal and ethical free/libre/open software for budgeting is https://skrooge.org (a program for GNU/Linux). It's not everything Mint is, but it's more private and, well, free/libre/open, and it's still a very capable full program.
Cool. I will need to figure out a way to get it working on Linux. I have used budgeting tools like Skrooge but it didn't make much sense to me. And before that I used excel to track my spending which ofcourse is the best way I have found to manage my money but it isn't the most user friendly way.