The VTSAX minimum was reduced to 3k a while back. If less than that, you can buy the ETF (ticker symbol: VTI)
Really, there is nothing you can do wrong here. Traditional, Roth, Brokerage, whatever, the most important thing is you are saving $. The boring details are just related to taxes.
Easy approach: 403b contributions Traditional, IRA as Roth, the rest in brokerage
If you want to dig deeper: compare your current marginal tax rate to your expected tax rate in retirement. If now > retirement, then Traditional, else Roth. It depends on income.