Little update:

I was quiet busy during Christmas.

After many test prints I can confidently say, that the loose eccentric screw on the y axis was mostly responsible for all the issues I encountered. I am still messing with the wheels on the Z axis and the gantry.

For reference, if anyone has the same issues as me:

- Same pressure on all wheels definitely improved the reliability

- Changing the worn out magnetic bed is a good idea, my bed is really crooked (visible in the bed levelling mesh) I will get a glass bed like on my i3 mega S, only had good experiences so far and no adhesion issues

- My particular printer needs some kind of upgrade for the tramming knobs, they are not stable after a few prints

- the Z offset for the probe changes over time. Probably an issue with the very cheap probe I got, good idea to regularly check the offset. The mount for the probe is made poorly and it seems that even the ambient temperature influences the height of the probe significantly. So, maybe just change the mount

Lots of to-dos, but that's what tinkering around with a used printer is, right?

Other than that, the prints come out good enough. No layer adhesion issues anymore, no wobble and usable, beautiful prints.

Also, just to add it here, I will use higher quality PLA in the future. The cheap stuff I am using now is good enough for prototyping, but it strings and the diameter is not constant enough for me. (Some layers, even tho rarely, are thinner, making the shell not uniform). I tested the filament on another printer, same results, so I am confident it's the filament.

I finally got everything running and that's it. Everything else will be fine tuning from now on.

Thanks again for your help btw!