Maxx already gave a good answer for when to use Guard Impact so I'll focus on how to play without it.

This is coming from someone who played semi-seriously for months before starting to incorporate GI in SC4.

The first thing to know is your character's fastest moves. It's often just a straight horizontal hit. After that it's a matter of knowing your opponent's attacks and knowing when there's going to be an opening. There's always going to be some time interval either in between or at the end of a combo where you can get in a quick swing,by which I mean you can safely do attack, not necessarily that it will land unblocked. Even if your counter attack is blocked, at least now you're on the offensive.

I've only played SC5 once so far, but it seems like Just Guard gives you a slight edge where you might be able to get in a quick attack before the opponent has a chance to block in some cases, but it comes down to knowing your opponent's moves and how to react to them.

Also some moves have a higher tendency to supersede the opponent's attack than others. I know in general a vertical hit usually cancels out a horizontal hit. Some moves are also faster than the initially appear. I know personally I've found moves that I previously disregarded as not useful turned out to quite useful as they are faster than their animation would make it appear. I think it's an issue of our mind typically watching the time between the start and end of a swing, rather than the time between the start and the time of impact, which may happen well before the end of a swing.

Finally there's a rule of any pretty much any competitive game that has a meta-game component: Set up expectations, then break them. Repeat a pattern until the opponent figures it out, then break the pattern with something that counters what the opponent's going to do to counter you. For example, lots of characters usually have variations on combos, so there's mid-mid-low, repeat that a couple times, then occasionally throw in mid-mid-mid. They'll go to block the low and get slammed with the mid at the end (bonus points if it's a ring out!). It's like in Starcraft, making your opponent over-commit to anti-air, then coming in with a big ground army instead.

How to Use 6K

First off - his 6K has been slowed slightly, yes. It's still an alright interrupt as far as speed goes, but it isn't safe, and it isn't great for moving your opponent (unless it's a CH¹). It is very good for ringouts if it is a CH, however. Take advantage of it if you manage to back someone up to an edge or if someone's close and starting something slow (to guarantee a CH unless they cancel). If somebody does seem like they're eating these, take advantage with a 44B (combo), or 6K into 1A.A (tech trap, I believe).

Actually Zoning

My preferences for for zoning (and a bit for punishing people at close range without actually zoning) are as follows:

• Crouch throws

  You mention throws being too slow, getting ducked, and interrupted. Astaroth's throws are actually fairly quick - if you initiate one after a block, it'll be pretty hard to interrupt. As for ducking, after a throw or too they'll probably try to duck (especially if you keep heaving on horizontals), which let you use Astaroth's even-better crouch throws. Alternatively, you can set up a good crouch throw off of a few moves.

  Setting up Crouch Throws

  • As far as distancing recommendations, one of my favorites is his 3K.A - it's a quick mid kick, followed by a fairly wide horizontal. It's not the best, though - the horizontal is pretty easily to duck.
  • Another good one is 6A - probably his best interrupt at close range. You get a second hit with 6A.A, and the second is a crouch throw setup on CH. However, it too suffers from the second hit being duckable.
  • I like 4A a lot at mid-range (it's slower) to build distance - on CH it's that same crouch throw setup.
  • Lastly, at midrange with their back turned, B+K sets up a crouch throw.
    
    

  Ruining Their Day with Crouch Throws

  Your options are 2A+G or 2B+G - both have BE² to take advantage of. Either do a pretty decent job distancing you from your opponent, ending with you at medium range with them on the ground.

• Command throw

  Astaroth's command throw 63214B+G (where he hurls the opponent straight up) is great for building distance if you follow it up with 22B or 88B. If you want to get real scary, use the 22B~BE or 88B~BE that knocks them up in the air again, so that you can follow up with a second 22B or 88B, 44B, or the A+G air throw. You're using his command throw as a launcher which lets you land things that you usually wouldn't be able to get away with at close range like that.

• Low attacks and quick interrupts

  Low Attacks

  • It doesn't build distance well, but 2K is great for quick pokes, and transitions well into his crouching 3K bull rush.
  • For ringouts and to make them afraid of horizontals, 1K.A is nice. Another quick first hit (this one's low) with duckable second, but it's very hurtful and distancing if the second hit is a CH. 1K by itself can be used interestingly as a low hit that leaves you crouching (ready for a bull rush, for when they duck the horizontal they think is coming).
    
    

  Quick Interrupts

  • Everybody loves 4B after they get used to it. It doesn't feel like you have advantage on hit, but try bull rushing or another 4B - it's weird, but it works.
  • 1B.B is great - it's quick, and on a CH you end up tossing them away from you. Makes for some hilarious ringouts, and could actually get you some okay distance on a CH.
  • Don't forget 6A and 6A.A, mentioned above.
    
    

• Bull Rush

  Astaroth's bull rush in this is insane. People literally eschewing his CE³ in favor of his 66K~BE - this particular attack provides a shield of some kind between the first and second hits. For any non-strong attack (which is most attacks they could pull off between the two hits), you'll see Astaroth flash red, and the attack will do nothing. It usually guarantees a second it if they try to attack, so mixing up the normal 66K and the BE is great. Plus, 66[K] (holding K) lets you mess with their timing, and if you hit with the fully held version it's a launch (if they're close).
  
  

_{Footnotes:
[1]: CH = Counter Hit
[2]: BE = Brave Edge, performed with A+B+K
[3]: CE = Critical Edge}