Short term cap gains are taxed as ordinary income. I am already filling out a 10-99 to write off expenses. So I even start an LLC or LLP or Partnership? Are the benefits really there?
I highly suggest talking to an account that works with self-directed traders who do it as a profession. This would give you an insight how gains/losses are treated and expenses around that LLC.
Deductible health insurance, better retirement plan options, you can "pay yourself" a stable income and qualify for income-based things like mortgages or HELOCs after a few years. Business expenses I think are the same either way, but not 100% sure. Personally, I'd focus on the trading. If that's going well, this stuff is small money compared to expanding or improving your trading.