In my experience, neither the Desecrate damage nor the Corruption damage is altogether impressive, and the self heal from Corruption is almost laughable. What really makes Desecrate obliterate enemies isn’t the AoE damage you can get on it but rather Galilea’s tendency to spin and attack everything near her as part of her standard combo while enemies are standing in Desecrate and taking more damage from everything.
The point of trying to get self-healing on Galilea is to get and keep her at max hp (because of the insanity of Dangerous to Go Alone). Generally, small amounts of life steal aren’t really going to do that, which is what Galilea is going to be getting from the Leechsteel Brooch because she just doesn’t deal that much skill damage.
What’s absolutely amazing for Galilea is her lore legendary (attack speed, hp regen, and 50% life steal on stunned targets; applies to any stunned target, not just targets she stuns herself) because it provides big self healing when she needs it (and she can trigger it herself by using her shield). Yes, it’s a bitch and a half to get (at least until they drop the lore fix patch), but it’s definitely much better for her, in every way, than the Leechsteel Brooch (you also don’t have to trust to the RNG to give you the loot you want; if you can find someone to collude with, you can take care of the Ambra kill lore in about 10 minutes).