The further you advance in the waves, the longer it takes to mine the immuried metal needed to make the columns. I don't know if this is intentional, but as a result, you spend more time to mine than fighting the boss when you're reach like the 20th wave.
To be honest, it would help if Oberon's Bounty worked for these ores, that would significantly help to solve this problem too
It's also very strange that oberon's is disabled for these ore nodes. That would mitigate the above scaling problem.
It's even to the point where Sheliak cannot break the ores in a single hit, so you get almost no ore available to build pillars at high waves (lv30+).