Hi - I have never used a Mara X2 so can’t really comment on how much of an upgrade it would be. However i did change from my Gaggia to a second hand Londinium and have never regretted it. The Londinium is very consistent and although a good shot on the Gaggia would be hard to tell from a standard shot on the Londinium it is the consistency that makes it for me. Even if the shot runs fast or slow I seldom have a shot destined for the sink. That and less tangible things like the look of the machine, the tactile nature of using the lever, the simplicity of cleaning and maintence, the simplicity of use and the joy of quiet pours (My Londinium only uses the pump to fill the boiler, not for preinfusion so only kicks in say once in every 5 shots).
I also dont know how much is just that the lever profile with declining pressure and temperature just suits how I like my espresso shots. I tend to like medium roast beans and the Londinium manages these very well indeed but you might need a different machine if you prefer light roasts in order to play with temperature, preinfusion and pressure to extract the very best from them.
I also own a vintage Arrarax Caravel lever from the 1960s which makes superb ristretto shots so it could just be that levers produce coffee how i like it to taste!
Before plumping for a machine I would recommend getting some hands on experience with different types of machine, possibly at Bella Barista if that is near you, or at a forum day if any of those are ever arranged again in future.
You could also ask if anyone near you has a Mara that they would be willing to let you have a play with. Worth a try!