Alright. Found it! Just like many of you said, it was the brake booster hose.
Found the brake booster after some searching and YouTube videos and checked it tonight after work. There hose is incredibly brittle and fell off of the connector. I’ve reattached it as best I can for now, and will be ordering a new one ASAP. Glad to say that reconnecting the hose fixed both the stiff brakes and the rough idle.
Thank you to everyone who commented and helped me sort out this mess I made.
Now to see if I can fix the broken speakers. Not really looking forward to pulling the door panels off.
Also of note: my local wreaking yard DOES carry my vehicle. Not sure why I thought they didn’t. But they have plenty of broken cars for me to dig through. Might be hitting then up for a new door panel and power window switch.