You simply can't beat MSC generator. It is cross-platform, full featured and well supported (Zoltan generally replies very quickly to emails requesting support or new features).
It takes its input from a plain text file:
Read the excellent user mnaual and search for "loop".
Really, the only app that comes anywhere close [if the phase flip trick doesn't work, which it usually doesn't] is Izotope's Music Rebalance
It is only available as part of a larger package & starts at $399
Even at that price it isn't perfect. It works on some sources better than others. A demo of the full RX7 suite is available, so you can at least ...
Have you looked at kiki (https://sources.debian.org/src/kiki/0.5.6-8.1/). It's written in Python and uses wxPython, so likely could be run on numerous platforms. It's in Debian repositories, and possibly other Linux versions.
There is a bit of a learning curve, but Postman is great and you could do the stuff you want for free. I use it every day to test urls and api end points.
You can use variables in the url or in headers. You can have different sets of variables for different environments. You can do it really simply, or create tests to check for ...