I am back from the dentist.
Everything went well. It turns out the brown on the teeth wasn't decay but rather something called "calculus" and they just cleaned it off with the water sprayer.
There were no signs of tooth decay anywhere. Nothing more needs to be done. They just recommend I come in every six months for a cleaning.
I am very relieved. I had been picturing the worst case scenarios - root canals, thousands of dollars worth of work needed.
They were all quite impressed at the good state of my teeth given my very long period without seeing a dentist. They don't even know the worst of it which is I went years without brushing my teeth at all back when I was younger.
I am very lucky.