The y axis uses more power because of the 2 stepper motors so it is unlikely the x direction would cause and increase in power use and therefore a loss in energy available for the laser. In my experience it takes a lot of passes with very tiny adjustments to get the laser truly focused enough to cut in both the x and y direction with the same curf. Zax suggestion to use a newer version of grbl will help with the motion control but I assure you there wont suddenly be more power when moving in the x direction. my focus is a rectangle with the long side on the x axis so for me it seems to have less power on the y until it is focused. It looks something like this and my eye cant tell the difference between three smallest rectangles.
Hi Stefan!!! welcome to the forum. @EleksMaker-Z is the companies face on here. He pops on occasionally but for the most part it just us users on here.
On my first eleksmaker laser (mine first one is like your first one and also a year old) it is very rectangular. I used to cut squares out of 3mm birch wood. I would have to adjust the focus until the vert lines and horizonatal lines were even. I havent noticed it as much on my new 2.5 and 1.6 watt probably because I am mostly engraving these days and not trying to cut.
.1mm is about what you can hope for as far as resolution.
You can replace the eleksmaker lens with ones from ebay and amazon. Do a search for Collimation Glass lens. They come in 1, 2, and 3 layers. I have been meaning to buy some and try them out. one guy on bencutlaser.us bought some earlier this year. The guys who were testing them said they saw an increase in power using the 1 layer collimating lens but the focal length was shorter so they had to move the piece closer to the laser.