This one should be feasible because other games have a similar mechanic.
People should be allowed to reroll crown crates by spending increasing amounts of gems.
If you want to limit it, make it so they can only reroll if the collectible is a duplicate. But honestly, the goal here is to reward whales, and make them feel seen and valued so, I think they should be allowed at least one free reroll regardless of crate contents.
So many whales have a surplus of gems and yet still don't have the item they want, it would make the crates still seem valuable in the long run to them.
I also think it's ridiculous that people can't just plop down money if they want to buy something straight, seriously, make the radiant aprx items available for 2,000 gems unless there's a sale for 500 or something.
Also another solution for what to do is convert crown gems to crowns or gold.
Seriously, something is better than just preying on people with impulse control issues and addictive purchasing patterns.