You are choosing to pay more tax later on a much larger sum of money. Most people’s effective tax rate when they file is somewhere between 12-16 percent or less on a single income family. look at your taxes the last few years to see what have you paid. Lowering your tax now is extremely short sided when you’ll pay 20 percent later on a much larger sum of money, plus at 70 you have to start withdrawing it, and your kids, if you have any will have to pay taxes when you die.
You can only contribute 6k to a Roth outside of the tsp which allows $19500. The benefits far outweigh not paying the tax on the money now.
edit: also if you are above around 200k income combined I forget the exact number, with a spouse you can’t contribute into a Roth outside of the tsp.