As we’ve said before, traditional knight training would last about 14 years. This has nothing to do with the availability of the materials and everything to do with the amount of time needed to train your combatants. If they’re just heavy infantry you could probably drop that to a year or two, but, that’s not really an issue, because you’ve got a few world building problems.
First off, meteors don’t contain a lot of iron. Meteoric iron was a real thing, and it did have some characteristics that made it desirable, but, this stuff is rare. For a meteor to deposit enough iron to really alter a nation’s economy like that, you’re looking at an extinction level asteroid strike minimum.
This means the strike either occurred millions of years before, or your city in the sand would have been splattered. If it was millions of years ago, then the asteroid would have been mined out long before the technology for full plate armor existed.
This is the other problem. Even if you have an infinite supply of metal, you still need smiths to produce it, modern drop forged steel is still at least half a millennia away (and would invalidate the advantages of actual meteoric iron), so that means your city will need massive numbers of highly skilled smiths working constantly to produce the armor. You can’t simply mass produce this stuff. In a pre-industrial environment, each weapon and piece of armor has to be handmade. This takes time, and a lot of effort. Removing the material availability doesn’t help you that much. You have iron, but you still need smiths.
This creates a massive knock on effect. You have smiths, so now you need to feed them. You can grow some food in the desert, probably, but you need a massive agricultural base. Egypt works as a possible model, the Nile created an incredibly fertile zone along it’s floodplain, but that’s not really a desert, or at least, it wasn’t. Deserts have a nasty habit of spreading, and chewing up more and more land over time. Most ancient civilizations didn’t actually choose to build in the deserts, it’s just, over time, the deserts decided they wanted to hang out with everyone else.
This is of course assuming the meteor didn’t contaminate the ground water, which is a real possibility. Water is a critical resource in a desert, and one you really do need for everything. You need it for the smithing, for the agriculture, and to keep people alive. The smithing doesn’t require potable water, but the rest does.
No water, no food. No food, no smiths. No smiths, no armor. No armor, no knights. No knights, no way to keep someone else from taking your chunk of space rock at blade point.
Of course another huge issue is that when you mix full plate with mid day desert heat, you end up with an inventive way to broil your own troops. Just, food for thought.