See Lake of Cobalt.

Probably doesn't need to ETB tapped; while it's a triple land, the downside of using it is pretty harsh, and it's otherwise a single colourless-mana land.

I agree that Arcane Sanctum may have been too good (more because it made designing other triple lands difficult. I don't think it was too good for power level reasons.) So I don't normally use the Alara lands as measuring sticks for triple lands. But Legendary Lands? That just doesn't feel right.

Quick aside: 'Monument to X' sounds like an artifact. I presumed this card cost 3 mana until I doubled back and looked at it again. In fact, turning these into cost artifacts might solve your problem. I'd normally say "five undercosted mana rocks is dangerous in today's Standard." But Legendary mana rocks? Maybe that's okay...

Note - previously this was exile from hand. A major cost. Exiling from library is a much more minor cost, almost free.

I know. I couldn't find a middle ground. Maybe graveyard? but that wouldn't work in every deck. And I don't want to limit the colors this appears in... Target graveyard? that is bound to work often.