PCB design of the copper area of a (linear) power supply
You have a big area under your capacitors that will certainly have ripple currents so, you have to decide where in that mass of copper is to be your star point. It doesn't too much matter where you decide that to be. It can be along any edge of that copper but. clearly it cannot be inside the copper block.

Once decided upon, stick to that. There will be no ripple currents flowing from that star-point unless you break that single-connection rule. Whatever ripple currents circulate in the big copper area won't circulate on (or from) the star-point connection unless you break the rules.


  Is it a good idea to put the star point onto the board of the main
  PSU? Or is it better to connect everything including the main PSU
  somewhere externally using thick wires? Will this have better immunity
  from ripple currents?


Using the main PCB (as your 2nd picture begins to indicate) should not be a problem. Stick to the star-point rules (just a reminder). You can use wires or you can use PCB tracks. Stick to the rule (there I've said it again).


  I used relatively big areas as power lines and high current lines
  (yellow areas, back copper) instead of thick tracks because of high
  current. Good idea? Will it cause any issues with capacitance? Should
  I use more decent/thinner power lines?


I have no idea what you are saying or asking in (3).