I find some of the easiest ways to think about Astra utility is breaking it up into Smokes + Everything Else. Smokes are fairly simple, you know all the choke points etc. Then for your other utility think of it like flashes or any other spot utility. Get used to placing 2-3 stars at the beginning of the round and placing the other 2-3 as and when you need them. See your teammate pushing, stun a common angle etc.
Stars even have a lot of power if you don't use them for anything, just putting a star somewhere can be enough to stop someone holding an angle in case you pull/stun etc. The stars are very much their own ability in themselves due to the threat of your utility.
I'd say my biggest piece of advice is don't put all your stars down straight away in the pre-round. Make sure you've got a couple to hand to react with.