So I had recently desynthesized more stuff. I had to also desynth with Goldsmith skill (and Culinarian skill), or leveling up my Blacksmith Desynthesis can randomly decrease it. So in the first session of Steel Doming Hammer desynth, I also did 100 Mythril Rings of Crafting.
The results are as follows:
It was exact tie, in term of Fieldcraft Demimateria I.
On my server, it is slightly cheaper to make a Steel Doming Hammer than a Mythril Ring of Crafting, due to the price of Earth Shard vs. Wind Shards.
So, the next day I desynthesized more Steel Doming Hammers, using the leftover Steel Ingots and Oak Lumbers from previous session, and also bought more from NPC.
Results weren’t as pleasing. Only got 15 Field. Demimateria I instead of 18. Just tough luck, maybe.