To put things into perspective - we have ten completed megaprojects, two of those Tier 2, so let's say we have twelve.
Out of those, six were built in response to a crisis (Both tiers of Sacred Forest, both tiers of Law, Palace, Census).
Four were actually prototype extended projects, so pretty essential for infrastructure purposes (Saltern, Garden, Temple, Library).
One was a super unique opportunity thanks to a Heroic Mystic (Sacred Warding).
And we are left with grand total of one megaproject, Canal, which I think was build in ancient days before we even knew what megaprojects are (though maybe Sacred Forest was the first?), and also in response to a big challenge, if not exactly to a crisis. And maybe it's also prototype extended project? Pretty situational one, then, so doesn't really count.
So no, I don't think we were building too many.
Arsenal is for the storing and organization of weapons
No, Arsenal and armory is not the same thing. Arsenal is a research and production facility, built to equip king's armies with best gear possible. Exactly what we need for steel.