So I picked up some of the simple green cleaner, and overall I'm fairly impressed. Soaked for about 4-5ish hours, no heat or scraping , just a simple soak, and it worked WONDERS compared to isopropyl. Obviously it's not completely perfect yet as there is still some staining on the walls, but honestly this is good enough for now. Flavor and performance is largely the same as it was pre-clean, making it seem like the staining is an aesthetic issue at its core lol.
I'll throw it in a baggie with some simple green and run it in my ultrasonic cleaner as I bet that will help loosen everything up faster. But yeah, I can definitely re-assure that this method works at least well enough just with soaking if you don't feel up to torching your insert like meeeeeeeeeeeee. (Also, I regret not taking a picture of the bottom of the cup before cleaning, the difference was insane!)