Got some great streamers to play my game today (thanks to https://twitch.tv/ursagames and https://twitch.tv/elysiagriffin), and a lot of great feedback on my page. I posted an update to my game. You can grab it on https://jamish.itch.io/ground-0!
Lots of small changes, like making it more obvious when the boss is impervious to damage:

Version 1.1
* Gameplay Changes
* Bullets now bounce off the boss when you deal no damage, making it more obvious when it's invincible
* Boss bullets move slower (20% speed reduction)
* Your bullets can destroy enemy bullets
* Visual Changes
* Enemy corpses are now transparent/darker, and will always render behind other enemies/bullets, improving visibility.
* Enemy explosions render behind other enemies/bullets, improving visibility.
* Bug Fixes
* Hoppers no longer move sideways when you pull the dirt out from under them
* Fixed a rare crash when you die in the same frame that an enemy spawns
* Miscellaneous
* New pause/quit menu accessible at any time by pressing Escape
* Game window is now resizable
* Both Windows and Mac build are now built with GameMaker Studio 2
(Note: I left my original offical LD42 submisson on my LD page)