How about brass?
Brass is superior to aluminum, but not quite as good as stainless steels, all in terms of corrosion resistance, melting point, and yield and tensile strength. It's not a bad choice, but you shouldn't really have any problem sourcing a piece of stainless steel for not much more expensive than a similarly sized piece of brass.
Archaeologists find clay plates with inscriptions that survived even after Vesuvius eruption.
Sure, but the number of clay artifacts which have been destroyed or damaged beyond recognition or usefulness vastly outnumber those found intact. Clay is also a catch all term for a huge variety of different compounds which can have widely differing properties. Most will be fire resistant as you say, but most will also be incredibly brittle, and will break or shatter with even relatively minor impacts, which could lead to complete loss of your seed phrase. I wouldn't recommend.
The most important thing to remember regardless of how you back up your seed phrase is to have more than once back up. I only use paper for the majority of my back ups, and I am completely unconcerned if one is damaged, destroyed, burnt, incinerate, waterlogged, etc., since always have at least one more in a geographically separate location.