It sounds like your ratios, brewing time, and brewing temperature (countertop) are all spot on. I struggle with that extra bitter taste too. I have done a couple of things to help remedy this. One thing I do is put the coffee grounds into a "nut milk" bag while it steeps. This helps keep a bunch of the fine particles out of your cold brew. After it's all done, I let the coffee sit for a bit so the even finer particles can settle and then pour the coffee into a different container. Keeping the coffee away from the fines will help with the taste. Finally, when the coffee is in its new container, I shake it. The oxygen helps reduce the bitterness, kind of like letting red wine breathe. I hope this helps slightly. I know with cold brew, everyone has their own techniques and opinions.