It's pretty much decided: The server goes back to Arch Linux.

  It's too much work and too risky to upgrade my OpenBSD. I have to wipe
  the existing server and install a new image. But how do  I  know  that
  everything  works  as  expected? I cannot test this, since I only have
  *one* server. If things break, how do I roll back? I could export  the
  old  image  beforehand  and  then re-upload it. But this takes so much
  time.

  Or I could rent another new server and set up the  new  system  there.
  That would work, but it's expensive. :( And still a lot of work.

  That's just not worth it. I never had to worry about these things when
  using Arch Linux.