I use Sphinx a lot, and I wouldn't change a thing about it. The thing is, if you're meeting the condition, you're likely only playing it when you'll trigger the Unliving keyword effect for triple brick production, so you're already recovering most, if not all, of your brick cost.
I will often end up using it not because I need to discard something from the opponent's hand, but because I want the recruits I'll gain by using it. I have a few high-cost recruits attack cards in my Unliving deck since recruits tend to accumulate, but I don't typically have enough to cover the cost as soon as they're drawn. Sphinx is key to that. I also like that I'm recovering much of the gem cost if targeting a blue card since many Titan pieces cost gems to play. When I can't play Sphinx immediately, it's usually because I have too few gems on hand.
So consider me a dissenting voice. Sphinx doesn't need to do more to add to bricks. I'd find capping its gains to 20 bricks to be a gigantic, unneeded nerf.
(Also, the "max 20" is per resource, as outlined in the manual and the norm across all cards that use the terminology.)