@Rnkhch wrote:
...my understanding was that the federal corporate tax rate is 21%?...
That is the tax rate for C corporations. S corporations are pass-through entities (to their shareholders) and do not generally pay federal income tax. This is one reason why it is critical that you have that consultation (as @Critter suggested).
In the meantime, the following are a few resources for you.
https://www.irs.gov/taxtopics/tc429
https://greentradertax.com/business-traders-maximize-tax-benefits-with-an-s-corp/
https://www.investopedia.com/articles/trading/09/incorporate-active-trading.asp
