For a few months now, all features of tikz have not been working with LMTX. After help from Hans and a bit of trial and error, I think that figured out what was going wrong with the catcodes in m-tikz: Since the texcatcodes are different from what tikz expects, the catcodes have to be set inside \starttikzpicture (in addition to before the loading of tikz files, which was already being done).
I am attaching the working m-tikz.mkxl file.
I have tested this on my course notes, which make extensive use of simple tikz block diagrams and pgfplots graphs. Perhaps others who were having issues with tikz could test if this fixes everything and, if so, we can add this to the distribution.