Go Pure Communism
Since the goal is to be left alone, not be unnoticed at all, you don't need to hide the existence of the star systems. You just need to make sure there are no opportunities for outsiders who might want to come in. In a Capitolist star system, there is always the opportunity for outside investors to come in, establish or gain control of businesses, and make a profit. But when they show up in your star system with a great deal on a ship load of hyperdrive parts, there is no one with any money or trade goods to buy them. The government politely declines the offer and sends them back to where they came. If they try to settle down and start a business anyway, they are arrested and thier assets all seized for violating anti-profiting laws.
The arrested persons are then deported to thier native government after a nominal level of mistreatment to prevent international incidents while making sure that those outsiders are adequately dissuaded from wanting to come back.
So, the only way to make a living here would be to establish citizenship and let the government take care of you, but the government does not allow immigration; so, outsiders have no way of making a living or being supported here. So, visitors can technically come, but they can only stay as long as they are living off of thier own supplies.
With no jobs, tourism, or immigration opportunities you eliminate 99% of the reasons that anyone might want to come to your star systems. The for that last 1% that shows up anyway out of matters of curiosity or diplomatic opportunities, you establish visitor zones that you direct any incoming ships to. These would be isolated outposts away from any colonies where you can welcome guests until you have the opportunity to politely ask them to leave while keeping them adequately far away from being able to make observations or influences on your culture.
Where Data Manipulation Comes into Play
The key information you want to manipulate is your military and economic capabilities. If the outside world thinks you are too strong and poor, they may come after you out of fear. If you look too rich and weak, they may come after you out of greed. But, if they simply have no clue what your military and economic capabilities are, then there is too much risk in contesting your government's sovereignty; so, other governments will have no reason to believe that a war with you would yield any desirable outcomes.
Focus on Surveillance Technologies
The only real risk this leaves would be the spies and surveillance ships deployed by foreign powers. Spies should not be too hard to deal with for a space aged totalitarian government. Every citizen will be marked at birth for tracking their resource distributions. From there, its not hard to tell who is not marked to be able to spot a spy the second they step foot into a public area.
That means the only thing you need to fudge to make sure they can enforce thier isolation is to make sure thier ability to detect a ship in thier space is better than any other nation's ability to cloak/stealth thier way in.