Hey Antichambre,
your project looks really great. I just looked briefly over the datasheet of the TLC5958 and now understand your problem with the multiplexing. Using a CPLD to achieve a precise timing is an interesting solution.
At the moment I am thinking about building a dedicated step sequencer that is fully integrated into the DAW, but I am just in the concept phase. When things get more precise, I will definitely take a deeper look at your solution.
Best regards, Sascha