You could be my twin, because your story sound familiar ;) I even have a similar set up.
You could check the temperature setting of your Mara x behind the drip tray. The lowest setting is too low for a medium roast (at least on my machine).
You could also try to pull a turbo shot, Lance Hedrick has tutorials on that issue on YouTube.
Something that helped my results with “difficult beans” was to try other brew ratios, like , 1:2.25, 1:2.75 or even 1:3!
Good luck!