if you get outflanked and hit by an enemy main fleet from a side of your empire that is very far away from the front-line there's almost nothing you can do.

as Azrak suggested, a Vasari empire can put up phase nodes everywhere and effectiveley defend the whole empire with just 1 defensive fleet. this is a huge lategame advantage for Vasari and the other races have trouble replicating it.

TEC and Advent basically have to build their defenders in response to an attack. i would recommend adding 2 frigate factories to grav wells that you intend to defend and leaving 50-100 fleet logistics open just for purposes of building defenders on location whenever you get attacked. if you repel the attack you can go counter-attack with the defenders you just built or you can scuttle them for some of the cash back. the main idea though is that you can sort of 'teleport' your defense forces by leaving them unbuilt. you only solidify their location at the last minute when you know where they are needed. 

How many turrents do you build? You may find that turrents act as more of a deterrent than hanger bays. I focus on repair bays and turrents first. As a matter of fact I've not built hangers at all lately.

Here's a thread about playing against the AI that I've found useful.

https://forums.sinsofasolarempire.com/341840

Pre-entrechment, this is pretty much the idea for deterring fleets.  Against a real fleet, an extra 10-15 fighters are going to get chewed up fast when the opposing will most likely field 20+ fighter squadrons + bombers.  Add in the fact that the fleet can easily focus on your hangars to shut down your fighters permanently.  On the other hand 20-25 turrets with repair bays can take a long time to kill with their high hp and armor.  I think vasari turrets have the best ability to actually take down ships due to their phase missles, but the TEC last the longest with their higher armor and better repair bays.  Advent also get 8 more tatical slots so that's good too. Ultimately though, no defense in normal SOASE can stop a fleet.  I recommend getting phase jump inhibitors on front line planets so when you do have a fleet arrive to defend and it can defeat the invading fleet, you can keep them there long enough to do some damage before they leave.

If you do get entrenchment in the future, that would really help alot defensively and in AI games I find it really changes the gameplay quite a bit.  A upgraded vasari starbase with turret and repair bay support can really rip a hole in a fleet.

It's not in stores.

There's been talk about the three expansions being bundled together in retail when they're all out, but it's not set in stone yet.

(Hi Annatar!)

Nice - Will check it out.