Interesting nodebuster-like game.
The pacing is okay. I think it may be a bit too slow, especially the starting part. I think the price of early levels of upgrades should be lowered a bit so that the player can afford a new upgrade every run.
Many other nodebuster-like games have difficulty levels. If you go far enough in a run, you can beat a boss that drops special currency, and increase the difficulty level. Increasing difficulty level makes runs more difficult but also rewards more resource to buy more upgrades.
IMO bosses doesn't fit this game, but having difficulty level is ok. I'm not sure what effects it should have though.
The quest system feels out of place. I suggest removing them altogether, and maybe replace them with difficulty level system instead.
There's an oddball upgrade: increase the max number of collectors to buy. I think it's out of place too. I think it should be changed to give extra collectors directly, so buying it is immediately useful, not requiring further spending into collector count upgrades.
Feature request: allow the player to move around in the upgrade tree. Currently there is only zooming but not moving around.
I suggest adding different types of collectors. They can collect with special effects: Collect in an AOE, scoop-like collection, bonus mushroom spawn on collect, bonus currency gain but spawn no mushroom, etc.
The AI of existing collectors may need tweaking: They currently go to random mushroom. I think they should go nearest instead.
Overall I think this game is quite novel. It needs more content and polish, and is worth working on.