Constructions don't build experience (unless it is something that requires architecture, like bridges, but walls don't do it), so you're actually killing your XP gain with megaprojects most of the time.
(Also, engraving serves a very good purpose early on - an egraved dining hall massively cuts down on unhappiness. Even if your statues are made of nothing but microcline, a statue garden in a fully engraved hall can easily have enough value to give strong happy thoughts.)
I only have one legendary engraver because he's the only one who I've let engrave. The new guy is taking absurdly long to get up his first five levels, but once you get it that far, it's very fast, and once you hit legendary, it's probably the fastest XP gain you can do.
This is because the experience you gain is fixed - it's probably 30 xp per tile, although I haven't explicitly checked that. However, it takes a couple game days for a low-level engraver to get that job done (and they often cancel to get a drink), so they get a very tiny amount of experience... this, incidentally, makes it a poor job for off-duty military if you only want a quick layover, better to use bookkeeping for that. If, however, you actually manage to clock the several year it takes of constant engraving to get past the first few years, the rest of the levels are significantly quicker - the time it takes to complete the job swiftly approaches zero. This means that, although there is a sub-geometric growth rate in the requried experience to gain levels, your growth in terms of how much XP you can acquire per given period of time effectively increases what appears to be geometrically. This makes engraving possibly the fastest attribute-building skill, and as the only bottleneck on engraving time is movement speed, the more attribute gains you have, the more agility you get, the faster you even move!
As a cherry on top of all this, you can do what I did - make one of your initial miners your initial engraver. When he reaches legendary status in both, he will be a virtual superdwarf in stats, and be capable of transmuting mountainsides into long tunnels and huge collections of stones with a wave of the hand, then go on to smooth the whole cavern on the trip back, and, if you don't expect to ever need to knock down any of the walls or dig channels through the floors, you can send him back once more for engraving the whole thing. That's likely 90 xp per tile for a job that gets done almost instantaniously.
This is why you need to bring the legendaries back down a bit, and dabblers should probably be made a bit faster. The disparity is too extreme, to the point where the higher levels are, contrary to the sub-geometric experience requirement growth, actually much easier to obtain than the lower levels.