Hi, I’m also a screen reader user and recommend this setup. Table navigation is certainly doable, but what I’ve found is that messages that are pinned read their status indicators first before reading the message title. This is not a huge problem, but it certainly slows down navigation.
In NVDA — the screen reader that I use — the commands for tables require holding down CTRL+ALT and using the up and down arrows to traverse rows, and left/right for columns. So, having a header for each topic title is also preferable because it’s only one single key press. I also know several screen reader users who also have cerebral palsy, which limits the use of one hand. In such cases, having a single keystroke would also aid those people as well.