My dude, you still can, only slower.
I think it's pretty Lore accurate right now. Look at Askonia. When the Great & Glorious Philip took over that system, he had an entire armada with him, and he still had a helluva time holding it.
You, my son, are a cheeky rich-boi (who almost certainly got his money through at least half a dozen underhand ventures) who thinks he can just dump multi-millions, equaling the GDP of entire systems in the core, into setting up a planet or system that can economically and militarily challenge multiple factions.
I'm not going to draw hypothetical real world examples because if you're playing a game like this you probably have some imagination too, but just think what a country like…say… Australia, only fully militarized popping up in less than 5 years and run by a single guy who has seemingly unlimited wealth, a powerful navy, excellent strategic and tactical acumen, and a shakey moral compass, would do geopolitics.
Use the abandoned stations and keep a low profile until you can afford to field a full defense fleet. Look for ways around the violent clashes, even if it's just while you gather your strength for the final hammer fall.
Use this as a chance to change your strategies for colonizing.
As a personal example, I used to look for worlds near the core to colonize, now I want them far, far away. Ok, I make less due to accessibility, but I now have 70-80 days or so to deal with fleets that try to harass me. And I use that time to jump them in hyperspace, where they can be split up and destroyed piecemeal.
o7
Actualy the easiest way to start is by colonizing rich 200% and higher hazard rich planets and keeping them at 3 population. It can bring you ridiculous money without making the whole sector go crazy but that feels like cheating.
Or just colonizing one terran world building a star fortress and letting everything crash against it as you kill everything as it tanks.
Your arguments are sound except all factions magically pull largest fleets you will ever see in the game before or after. Where are they when they get raided by pirates or other factions or when they leave their own systems unprotected for the entire game.
Only Sindrian Diktat has decent fleets protecting the system and that place is a glorious warzone you can scavange a brand new fleet from.