Just a follow-up on this issue as I've found the fix, and perhaps it would be helpful to others (now or in the future). The spring washer (part 173 in the diagram above) on my base seems to have completely flattened over time. This is what introduced the rocking in the depth stop. I replaced it by doubling up on a replacement washer (Bosch Part #3-916-060-500) which resolved the issue. Pretty simple, and wish I would have taken the time to figure it out previously.