Really not sure what your question is here. There's no point in contributing more than $19.5k because that's the yearly maximum (for 2020; it goes up)—you aren't allowed to contribute more. You can choose to contribute Roth or Traditional.
Obviously index funds track the stock market but you're not playing individual stocks, that's what I meant. Do you know about the "
lazy three-fund portfolio"? (For TSP you need four because you need the C and S to get the whole US market.) I'm young so I don't have anything in bonds at all, right now my contribution is split 67% C, 13% S, 20% I. That's weighing my portfolio more towards risk without doing something silly like buying individual stocks. You don't need to stay in the default lifecycle fund.
If you want to make the argument that there are more immediate concerns (paying off credit card debt or saving for a mortgage down payment) then sure. Or if you are concerned about retiring early so you're setting something to draw on before you can take TSP withdrawals. But I don't know why you would generally prefer to put retirement money in a brokerage account before maxing the TSP.