It's not really clear what you're asking and most of these questions can be researched individually, but I'll give it a shot:
What would Earthlike planets bigger than Earth with <10 m/s^2 gravity
and complex life be like?
Large is bigger than Earth. Assuming that such planets exist (though I
guess if such planets don't exist that would explain much of the Fermi
I'm not sure I follow the Fermi Paradox argument cause life probably could develop on small earths just as easily as large earths, though tectonic plate movement, a core maintaining a magnetic field and atmosphere retention are all factors, so there are sets of circumstances that would be more likely than others but there's no direct relation with one specific type "big lower G earths" and the Fermi Paradox.
But, to answer your other question, there's no reason why a lower iron, larger but lower gravity earth couldn't exist. How common they would be depends on how common Iron is in that the gas cloud that forms solar systems but lower Iron content is certainly possible. There's other ways too, for example, a giant impact, like the one that formed our moon could form a low Iron core high mantle planet. How common a larger but lower G earth would be - hard to say, but I'm sure there's at least some of them out there.
Maybe some amount of iron is needed though, to make a magnetic field.
Some iron is likely inevitable as it's such a common element released in supernovas. Of the 4 inner planets, only Earth has a really good magnetic field, and all 4 inner planets have iron cores, so clearly Iron isn't the only factor in a magnetic field. You also need the right temperature and pressure and, I think, inner core rotation and probobly some measure of radioactive elements to keep it hot.
I guess you could also have a planet like that with very good climate
and gravity for Earthlings but toxic air
Toxic to whom? If life evolved on a planet, they would likely adapt to whatever elements were in the air. Oxygen used to be toxic to early life when it started to collect in the oceans once the dissolved Iron has been used up. Early one-celled life first protected themselves from Oxygen, then, later, began to use it.
and a lot more carbon or ammonia or methane or hydrogen or
formaldehyde or acetone or alcohol or hydrocarbons than here.
There's too much to do here as each element could be written up separately, but one thing to consider, while complex elements like acetone or alcohol do appear to exist in giant gas clouds in space, and they likely exist in comets too, when a planet is formed, there's enormous heat in the process. Anything complex like that is likely to be destroyed in the planets formation.
Also, photosynthesis, if that's going to be the basis for lush life, that creates Oxygen and Oxygen is very reactive, it would bind with any hydrogen and free Iron and likely free Sulfur and other elements and it would in time, react with and remove any CH4 from the air. If your "lush" planet doesn't operate on photosynthesis, then you can set up a different set of rules, but Oxygen is very reactive and certain other elements aren't possible once Oxygen becomes present in the atmosphere. Oxygen from photosynthesis would react with and remove any fairly large amounts of acetone or alcohol if it was present on a planet.
Water is also very common in the universe, so, unless the planet is below freezing, or very high pressure, or very hot, it's hard to imagine oceans of anything other than water. On titan there are oceans of Methane, but that's cause it's really cold. Oceans of alcohol is a fun sounding idea, but I think, unlikely.
It wouldn't be Earthlike but it'd look like a lush land where the
rocks are all graphite or diamond and the sky is brown or something.
That planet could be like the Garden of Eden for a sentient that's
mostly acetone and would get drunk from an ounce of H2O (or whatever
chemicals are appropriate).
Graphite and Diamond are formed by high pressure inside a plant, so all the rocks being graphite seems unlikely. More present - sure. All rocks, I don't think so. A Brown sky - sure, why not.
"a garden of eden that's mostly acetone", because acetone is both somewhat complex and fairly reactive and it contains oxygen and because most primordial oxygen on any planet would probably be bound more simply to hydrogen or Iron or Carbon, I have a really hard time seeing that as a likely outcome. Unexpected combinations could still happen, so I like where you're going with this, but that specific example seems unlikely.
If we take the assumption that life forms in water / needs liquid water, it's hard to figure that water would get anything drunk. It's too essential to life. Our bodies are 80% water. I don't think it's possible for life to develop where the body is 80% alcohol for example. An Alcohol water-mix, kind of an anti-freeze of sorts on a colder planet. . . . who knows, maybe. On a cold enough planet, cold blooded animals resist freezing by limiting the amount of water in their bodies, so some exotic outcomes are possible, but no water, I have a hard time seeing that.
We've only got 1 example of the evolution of life to study so far, so how it might work on another planet with a different conditions is far from exact science. There's always going to be certain parameters defined by the chemistry, like carbon based life is probably the only one that works, other molecules simply aren't able to do what Carbon does and liquid water may be essential too, but there might be a lot of exotic and unexpected ways life can happen within those parameters. Too many unknowns to say for sure.
Hope that helps some. At least, that's my 2 cents on this question.