Earned income is required to fund most types of retirement accounts.
If you form an entity for trading, and you and/or your entity does not qualify for trader tax status, capital gains from trading will be passed through as capital gains on a K1, which is not earned income.Nopw there are exceptions, but we are talking about trading for a living, not a hedge fund, not a portfolio manager, or other things, just trading for a living.
Trader tax status is what allows for the earned income necessary to fund most types of retirement accounts. Applies to sole prop, Schedule-C traders too. In that case, the trader must have trader tax status to successfully use specific techniques converting schedule c business income(cap gains) into earned income. By the way, no formal entity need be created for Schedule-C traders, individual or a de facto partnership... optics suck, done that way, imo. But the point is, "forming an entity" as you said, is not the key, earned income is.