Maybe make:
ZoC 1 tile
Can be destroyed by anyone with or without war.
Cost more with each cast
Increased upkeep per existing monolith, Or limit the times it can be cast through out the game.
It really needs to be something to the tune of 1 upkeep on top of 20-30 mana for the cast, and limited to 1 casting per turn. That puts it at a time cost of approximately 10-15 turns, which is roughly equal to the early game time cost of a pioneer, plus the benefit of not using the production queue.
As an alternate to the "once per turn", if it were capped at 1 monolith plus 1 per shard controlled, it would make it largely a shard acquisition device, yielding a net 1 mana per turn for the caster, and helping further identify Pariden as a magic-focused nation.
I'm torn on the 1 ZoC thing, since having 2 ZoC does help limit encouragement of Pioneer spam, but it also devalues cities.
It is hard enough to keep your outposts as is, though. I would not like to see them as something you could break without declaring war. Otherwise the AI would break them just because they could. Heck, I know I would. Every time.