I’m horrified that you’re using Piefed for work.
Have you considered using Piefed client that has „mark all as read” button or „mark as read on scroll” features? Voyager does this, I’m pretty sure many others do. Another approach is to browse content sorted by new. In such setup all you need to know is if content is above or below your scrolling position. Hopefully these can make your work less tedious.