My kids have a ton of legos, it's their favorite thing. However, and this is the important part: you have to let go of the concept of a set.
Keep new assembled kits out, let them play with it as built from the instructions. But then as it falls apart with play, and the kids don't fix it the same way it was originally built, it eventually goes into a big box of former kits that are all jumbled together.
We did this, and without prompting to do so, the kids started building their own things out of the box, exactly as you did with your kits.
You just have to learn to let go of the set, and it becomes exactly what you want.
Edit: I'm not sure if a $800 set has that same property, but for the everyday $5-$40 sets, absolutely treat them as temporary collections, and life is great.