Underground Structure spawning issue, how to give airspace a higher priority?

Hello folks, I'm having a bit of an issue with an underground structure that I can't seem to logic out the answer to.

basically I'm spawning a very large custom geode. my issue is this, if i have it spawn during world gen it tries to spawn before all the chunks exist so it ends up being cut off and not looking very good.

But if i spawn it in after world gen with a block that randomly ticks then spawns it in and removes itself, then I end up with it spawning after caves so it cuts them off. 

what I actual want is to be able to spawn in the whole thing without it being cut off but i want the caves air space to have a higher priority so it ends up caves and such generating through it. Basically 'replace any block accept air' if that makes sense. I'm open to any ideas for how I can achieve the look I'm going for, I've tried a few things so far but I can't seem to work out how achieve it on my own.

I tried adding some air only structures as well and having them generate via the same post work gen method, that didn't work either as with random ticking I can't chose to have all the geodes generate first then the air spaces. Any ideas are greatly appreciated