What to use for a matcha whisk?
The bowl isn't absolutely essential; it's part of the aesthetic. The shape does serve a purpose, as most of them have nicely curved bottoms that will minimize the chance of the powder from clumping near the edge of the bowl.

I've used a normal whisk when preparing "matcha latte" drinks for a client of mine back in the day. There's an advantage to having the 80-120 "tate" (bristle?) because they make it easier to break up beads that form as moisture contacts the tea. But a normal culinary whisk will do the trick, perhaps with a bit more work.

I don't see how the silicone brush would help whisking; they tend to be pretty floppy (and mine has occasionally had some of the silicone strands break off when I've used it for brushing food).