Thanks for the feedback!
The steam -> hydrogen recipe is 1000 steam to 500 hydrogen (with max prod it becomes 1:2, i.e. lossless). I think another mod is changing the recipe; maybe Muluna? Though I believe Muluna will stop changing gas recipes in an update soon.
You can think of the recipes that use activated hydrogen plates as shaving off tiny fragments; but because it spoils in only 1 minute, it's impossible to shave off enough to deplete it in so short a time. In effect, every recipe does give you (usually less than) 99% of your plate back, because it's always more spoiled. The mass is restored by the stabilization process.
The original versions of the ammo and thruster fuel/oxidizer recipes were basically what you describe, but it didn't really work. It required just as much infrastructure on platforms as the regular recipes, and the production bonus was never enough to justify the increased risk of explosions and the complexity of hydrogen plate logistics. And especially before you've researched advanced hydrogen metallization (which requires cryo science), plates are very expensive. However the ammo and thruster fuel/oxidizer recipes are the ones I've found hardest to balance and to playtest, and they're the most likely to change.