I've been skimming through this thread, and as others have mentioned, the acid was the first thing I thought of as well. My mouth can be very sensitive to acidic foods. The first couple of summers that we had a tomato garden, I developed sores in my mouth from eating too many tomatoes! It was my dentist who helped me figure it out. And she recommended swishing maalox around in my mouth, which sounds worse than popping a Tums, yuck!
I've never had mouth sores from coffee, BUT I do have problems with acid reflux and heartburn. I have tried many different kinds of coffee with different levels of acidity, with varying results. I am not a big fan of Starbucks cold brew, mainly because I don't like the taste. It almost tastes "fruity" to me in a way I don't like.
One thing I did have some success with was chicory-blended coffee, New Orleans style. Brands include Cafe Du Monde and French Market (
https://www.reilyproducts.com/produ...edium-dark-roast-coffee-chicory-12oz-red-can/ ) Do a search for chicory at that website and you'll see lots of different options.
They also make a cold brew concentrate that you can mix with cold or hot water (or warm milk to make Cafe Au Lait, yum!)
https://www.reilyproducts.com/products/french-market-coffee-cold-brew-concentrate-classic-chicory/
Or you can buy just the chicory and mix it with other ground coffee of your choice (
https://www.reilyproducts.com/products/luzianne-chicory-coffee-partner-6-1-2-oz/ )
It does change the flavor of the coffee somewhat, so you'll have to see if you like that.
Having said all of that, I've had to give up coffee altogether as my GERD has gotten worse over the years. I have a cup once in a while, but I can't make a habit of it or my stomach gives me trouble.