Even if you keep the 9k steel of a colony and make scouts it will still run out in a month and you can just sit 5 cruisers on it.
Just regular stuff with colonies and mind control and things.
Which is why using 1 arc for a colony is stupid, even more stupid when you are fighting. I've got colonies that hover around 8-9k steel. I've found that given the initial resources last long enough that by the time I'm running out I've gotten to the point that I generate 1 scouts worth per turn. That combined with the fact that for some reason some turns the scouts survive allows me to keep those planets defended indefinitely. Mind you the planet has never been society level 1 on the attack, I think the lowest was about level 6. The bigger threat is if your enemy can encircle you and prevent Arcs from breaking out.
With regards to automation I am making scripts to check for planets that need Mind Control, confirm that mind control system are active, flag planets that have inactive upgrades in general. My next project will be to set up automatic builds. I'm hoping to have planet categories and for each category there will be a build routine, along the lines of start upgrade x when planet hits society level Y. I would like to add some more checks too like ensure that population is a certain level, or income has reached a certain threshold.
Yeah, the massive strength of some empires is frightening. I've considered moving a dozen arcs across the galaxy and coming back next year, but that's too long even for me.
I don't know how much PDs can even the odds, I bank on them, that's why my central regions are covered in several of them. At the very least, it's going to be very expensive to attack me.
Pick somewhere close, but empty might take a few months to arrive, be aware that once you set up shop though the starting algorithm may place new players close to you. PD's even the odds nicely, though that is my experience with also using scout defenses, when Flyingmana attacks planets in my developed areas he usually targets small planets. I use a scout to defend the planet, build a PD near by, whittle down his fleet and then mop up. I think I have a tactic that gives me and advantage against PD, but I need to test it further, and small defensive fleets around the planet would still prevent you from making any real gains.