Hockman, I spent some time sourcing 16 x 16 x 3/4" silicon carbide and, so far, I'm coming up short. I did crunch some numbers and, from a thermal mass perspective, I really believe 3/4" is necessary. Even though member November used silicon carbide years ago, from a NY style perspective, it's still a bit new. If you can wait, then I might have better luck sourcing it, otherwise, I'd go with steel.
I don't think steel can do Neapolitan in an unmodded oven. Silicon carbide (SiC) might, but the jury's still out. Honestly, Neapolitan in a home oven is a bit of a crapshoot. If you've got a really strong broiler, then you might be able to pre-heat 3/4" SiC to 550 turn on the broiler and, maybe, achieve a sub 2 minute Neapolitan bake. Maybe. If your broiler is weaker, which, from what I'm seeing applies to most people, then you've got to do a trick that will push the oven temp higher, and aid in top browning, but, if you're working with a conductive hearth like SiC or steel, that higher temp might burn the bottom of the pizza, so when you get into oven mods, it can actually sometimes be helpful to be working with less conductive stones, such as the cordierite kiln shelf you were looking at. As you get into modding, though, you have to make the decision of how high you're going to push your oven. Lower temps = easier on the oven. Higher temps = harder on the oven. If you're willing to go very high (700ish), then you want one of the least conductive stones you can get, such as fibrament or quarry tile.
Neapolitan in a home oven is like trying to find your way through a maze. Two people out of maybe 50 to 100 have obtained the holy grail. A lot of it boils down to luck. I've started looking at broiler wattages/coil arrangements in hopes of measuring broiler strength, but that's really no guarantee. Getting Neapolitan bake times in anything other than a WFO is proving to be especially difficult. On the LBE side, it's like 4 people out of maybe 100s.
If you can find out the wattage of your broiler and take a photo to show coil arrangement, then I might be able to give you an idea if pursuing Neapolitan is worthwhile, otherwise, NY style is SO much easier. 1/2" steel plate and you're good to go.