Fire is a process, not an element. You concentrate more energy into a small space and that energy will build up as heat and presto: fire. That's easy as in "lifting an engine block" easy. You need a lot of muscle to pull it off, but it isn't complicated. That's why, as far as Harry is concerned, magical fire behaves as regular fire, as soon as it is summoned.
The same goes for wind, as that's just moving air around. Lots of power, not a lot of brain needed.
That's not true for water or earth, those are actual substances. Creating them as seemings (ectoplasm) is easy, because ectoplasm is malleable like that. The actual stuff is a lot harder. We are talking E=mc˛ times harder. Creating matter out of Energy is, more or less, the scope of a god or maybe an archangel.
Then again, we are talking about wizards here. Wizards cheat. So because you are very adapt at earth magic, you have a pretty good connection to some parts of the nevernever that are related to earth. While stuff from there would still be ectoplasm, you can use the place as a conduit. You can prepare a pair of gloves to allow you to rip open a small opening that will always connect to the same place, because you prepared the gloves for that. Now there's a spell construct in place that opens another portal back to reality to a place you prepared as well. From there, you can draw anything earth related to you.
Maybe the second glove acts as a sort of dial that allows you to get the second portal close to whatever material you need, as long as you prepared it.
It would be a lot of work to prepare, and I'm not quite sure if it's actually worth it, but it is certainly doable, if you are dedicated enough.
Ice can be water magic, but it can also be fire magic. Fire deals with focusing energy into one place. If you take the energy from one place, that place will freeze quickly. Water is about dissipating energy. If you dissipate a lot of energy from one place into its surroundings, that place will freeze.
Harry does pretty much that, by the way. Only he doesn't use conjured water, just water vapors from the air, which is almost always there, anyway. He just condenses it around the bad guys and then freezes it, all with the same spell.