I'm only going to bring this up once, then assume you're on the default option (non-lazy)
There is a toggle in the game setup lobby called Lazy AI. By default, this is off, and the AI will, among other things, camp out a distance away from the wormhole. With it on, the AI will camp out directly on the wormhole, which makes a considerable difference.
First up, scout pickets will help you locate threat buildup. This is very worthwhile information, and I've bound the galaxy filter for threat to the T key to expedite things, although I can find out some of the same information from the Threat tooltip.
Second, a threat fleet will attack it's target if it feels your are sufficiently vulnerable. This is why it tends to build up to a horrifying mass outside your chokepoint. If you're worried about two large but seperate groups of threat joining up (particularly during a CPA), it can pay to load your mobile units into transports and pull them a couple of systems away from the front line. There is a chance you'll bait the smaller force into attacking now rather after it joins up with the other seven hundred oncoming ships, and the transports are fast enough you should be able to return to the scene of the crime in time, especially if you have raid starships or spider turrets or tractors/forcefields placed specifically to prevent or slow retreat.
If that isn't an option (and you have auto AI progress on), drawing them to the wormhole early with a sacrificial fighter fleet and then hitting them with a lightning warhead while they're still on the far side of the wormhole can save you a huge chunk of time rebuilding. It's still much better to do without the AIP increase if you can manage it. I particularly like to send in the sacrificial fighters, then send in either Spire Tractor Platforms or Riot Starships MK1 (with tractor and shield) at the last moment, wait half a second for the tractors to engage, and then immediately pull the abducted ships back to my system. For the best results, pay attention to the location/existence of your rally points, and consider leech starships/mercenary parasites.
In theory, if you send in a small enough force, you won't agro the entire system. You can often send in just a wave of mark one fighters and take out Tachyon Sentinels without causing a huge issue. Going near or even targeting (from the other side of the system) the AI command station will have dramatic results. Blobs will usually agro things. Sending in just enough fleetships to take down one post at a time will do a much better job of minimizing resulting threat. In practice, guardians and starships (yours and the AIs) make it a bit difficult to pull off that balancing act. And, as you take down guard posts and their defenders, the equation will skew that much more towards agro.
If you're just passing through a system, you can probably minimize threat with the use of (Assault) Transports and Cloaker Starships. Putting units on "Standby" so they don't shoot anything other than their primary target can also be helpful, especially if the balance of strength in the system is already quite close.
Another thing you can do is 'sacrificial systems'. Your choke points and whipping boys are always heavily fortified, so you're more likely to have huge threat buildups. Having an expendable system outside your defensive line with light defenses can help keep threat buildup low.
Various minor factions can stir up trouble and loose threat on the galaxy (Dark Spire for example can take out guard posts and also cloak). The devourer golem, on the other hand, will lovingly eat both your fleet and the AI fleet and leave little left for threat. In a more 'vanilla' game, you need to keep an eye out for any tractor units the AI has on hand, as they will love to haul your fleetships deeper into their territory, stirring up trouble and alerting systems along the way.
If I am going to maintain a particular border for a good long while (preferably away from any MKIV planets), I neuter the adjacent systems, taking out guard posts. While the guard posts have an impact on reinforcements, I find the reduction in guardians most significant. Then, I go to the "CTRLS" menu in the bottom left, switch to the Planetary controls, and put a high "number of enemy ships" (depending on the mark of the planet and my fleet) and a lower "number of aggressive ships" (depending on my defenses, AI bonus ship types) alert on on that AI planet. The first will warn me if the regular defensive forces are growing too large for me to handily clear out. I might set something like 500 for a mark two planet, for instance. If this alert goes off, I need to take action while it is still convenient to do so. On that same system I might set an alert for 100 aggressive ships, which might not require immediate action, but will remind me to make sure I have transports ready, and to only send my fleet on limited engagements where I'm capable of pulling back if something starts going wrong. I'll happily send swarmers or fighters on suicide missions, and starships on surgical strikes, but bombers and frigates take more time and resources, so I need to be careful of fleet wipes there.
There is also the incredibly helpful "ignore this planet when auto-pathing" option.
Sometimes I actually -like- to knock a system into threat mode, particularly if it lets me fight outside the influence of a nasty guard post or fortress. Once it all gathers together, then I send in my blob!
Oh, and I nearly forget. There are human-buildable warpgates for fabricators, fleetships, starships, etc. They can be a big help getting reinforcements to a forward base without antagonizing all the systems in between if a transport pops, though this works better if you have unlocked "Warp Jammer Command Stations". A different option is the Mobile Space Dock - if you can replace your losses without leaving the system, you won't need to leave before you kill most of the ships.
There is something called border aggression, but I don't have an intuitive understanding of the balancing points for it yet at this current release.
