I haven't tried every WM, but I have tried a few and never had a problem. But, yeah, given your issues with your chosen WM and lack of use, I can hardly blame you.
I use a niche WM, so I'm kinda asking for trouble. I'm pretty sure I would not get any support from them.
I noticed that i3 WM has Mathematica specific workarounds. Might be an issue with non-reparenting WMs in general.
> Mathematica is one of the two pieces of proprietary software I use,
One is the other one?
SketchUp for woodworking plans. I've yet to try a few of the open source CAD solutions, I might migrate if I find a good one. For more artistic modelling I use Blender.
I also play games on Windows/Steam but I could live without that. Openttd and Nethack would do :)
But it's very buggy on Linux, and did not work out of the box last time I tried it. It somehow violates the X11 protocols (some window managers have Mathematica-specific workarounds) and in my case the windows were all over and mouse clicks didn't hit the correct positions. I can't justify buying $300 worth of software I need twice a year and I can't trust to work correctly.
I've used SymPy and Maxima but they are not as easy to use. I managed with SymPy to get some results but they were not as well simplifies as Mathematica results. Maxima was nearly impossible to use for me as I have no background in symbolic computation and the documentation expects you to understand the algorithms (e.g. different methods for simplification).
The biggest plus of Mathematica is the docs. They have tons and tons of examples and every function is thoroughly documented. And it is easy enough for me to understand without a lot of math background.