Have you tried boiling water & some good dishwasher detergent? Let it set for like 30-60 minutes. You should see the lovely brown chunks start floating ☺️ Otherwise, try a lighter grade steel wool & some Dawn dish soap.

I used to clean my mom’s thermal coffee pot for her using the boiling water+detergent method. Once I taught it to her, she started using it too. Worked like a charm & the coffee pot came out looking nearly brand new.