Layouting in plantuml is tricky, but I learned some tricks the hard way, which make larger diagrams more tenable. One core trick is:

  upper/left arrow lower/right

So e.g. you write:

  upperBlock <-- lowerBlock

instead of:

  lowerBlock --> upperBlock

This alone drasticly cuts on the amount of hidden arrows with forced directions you need to use in larger diagrams.

That said, plantuml could use a more explicit way of providing layout hints. E.g. an extra declaration that some objects form a grid. Something like:

  grid
    Comp1 Comp2 Comp3
          Comp4
    Comp5       Comp6
  endgrid