I've heard horror stories of budgies eating too much gravel and dying....they just don't know when to stop eating it.
If I understand it, gravel helps aid in the digestion, maybe you could offer a mineral block.
Not sure if they would go for it, but it's worth a try.
How is the clay working out?
Well, my older budgie (10+) used to have problems with grit, he would occainsionally puff up and look uncomfortable, fly to my curtain rod, and start emptying his crop of gravel, it would come raining down. And then he would feel better. So after the 2nd episode of that about a year ago I stopped giving it. Of course they would seek it out in houseplant dirt or wherever they could find it. So recently I found that clay-cal and offered that....I had asked the vet about it when I was there for Nika and she said it was better than gravel and better than houseplant soil. My older budgie never had an incident with the clay, but their droppings became like cement so I stopped. All the plants are outside now, so there is no access to dirt. I may just offer the clay very occainsionally just to satisfy their desire to eat earthy granules. I wonder why they bother eating gravel if its not good for them?
Sorry this post was a novel.