Tumbleweeds definitely aren't supposed to be instakill, sorry about that haha!
Part of me likes tumbleweeds as kind of this wall that you have to use your abilities and navigation skills to get avoid. I believe the way they're supposed to function is to only on the very bottom of them. This may speak to Mike's concern about 3D objects being hard to distinguish though...and while it's part of why I like and think it makes them different it can be confusing.
Maybe we try spawning single tumbleweeds that appear on their own throughtout the biome instead of focused dense sections? And maybe they only hurt you when they hit the ground/appear to land on you. Even typing this now I'm like brainstorming an almost frogger-like section or maybe challenge room that causes you to weave in and out of tumbleweeds. This could be extremely bad lol, but I think they have potential to create some interesting quick environment-reading opportunities for players and are a weird non-living entity that feel thematically nice to exist in the desert and could help create the identity of how one approaches and views the biome