Man that's just funny that den health outscales the mountain at that stage. Exponential scaling is tough. But yeah the demo kinda ends at lvl 50 and there is no actual refined endless mode yet. Things just keep on going and scaling way too much. For the full release I will try to make endless mode actually "balanced", though it's not gonna be easy with the way the game works. It was never meant to be the kind of incremental game where you keep going indefinitely and the numbers become indecipherably big.
Yeah mithril is only obtainable from RNG dens, even though it's a gating resource. I also don't super like how it tends to be better to focus on one unit, but besides in endless mode where you'll reach max upgrades, I'm not exactly sure how to best encourage players to invest into many units at once. It's an interesting design challenge.
Indeed the poor runners get heavily overwhelmed late game. Hopefully increasing upgrade tiers for all the mechanics you mentioned, plus adding two new retrieval units (Bulldozer and Stonecaller), a few retrieval rituals, and another type of protection unit, will be enough to keep things going in the late game and beyond.
Thanks for the kind words and great feedback!