I ran into the same problem and it is clearly a design flaw of the z-axis. Just try to move your axis up and down with a little force and you will notice some mm movement.
For resolving this issue a simple machine swap will be no solution, so I designed a z-axis stabilizer for the Elegoo Mars with little parts and without opening the housing and loosing the warranty. After installing my mod the problem is down from 1-2mm deviation to 0,1-0,2 mm, which is OK for my designs and may be further improved by careful zero calibration. The z-axis is now stiff and doesn't move along axis direction without compromising any machine function. If you're interested you find my build on Thingiverse: https://www.thingiverse.com/thing:3918220
Mike
For resolving this issue a simple machine swap will be no solution, so I designed a z-axis stabilizer for the Elegoo Mars with little parts and without opening the housing and loosing the warranty. After installing my mod the problem is down from 1-2mm deviation to 0,1-0,2 mm, which is OK for my designs and may be further improved by careful zero calibration. The z-axis is now stiff and doesn't move along axis direction without compromising any machine function. If you're interested you find my build on Thingiverse: https://www.thingiverse.com/thing:3918220
Mike