Okay, so I did small research and I think all problems are solved. A bit of information, maybe someone else will also struggle with similar problem.
1. Using buck/boost converters near signal wires (like probes) might induce noise and cause problems
2. If you struggle with unstable printer work/your voltage readings (in DWC) vary under load (even if PSU ratings are higher than requirements of your setup) check your PSU in diffrent position (horizontal/vertical), if it makes diffrence you probably have problem with cold solder joints - if you are familiar with soldering and electronics - redo solder joints in your PSU - if not, better not to do it yourself.
I did small research for my printer, decided to do some tests with opened enclosure of PSU to measure some things and noticed, when I test it laying flat, there is no problem (in printer PSU is mounted vertically), as soon as I mount it back, problem occured again.
Decided to redo all solder joints on PCB (I couldn't find anything that looked really suspicious) with special attention for filtering sections, added also some lead-solder. After that I don't have any problems like before.
I think problem is solved.