Countries need intelligence agencies -- they are the best way NOT to fight future wars. SIGINT is a useful function when used against foreign leaders. It's just a terrible idea when used against the population as a whole. Separate the parts that work and are useful from the parts that are destroying the country. Just don't lump it all together.
Unilaterally disarming is not a way forward. The only thing you accomplish by doing that is allowing dozens of other countries to spy on you without your knowledge. How is that a solution? These movements are in real danger of becoming "useful idiots" for others who wish us harm [1]
The only solution here is parity, i.e., whatever blanket surveillance is going on by corporations and the government regarding every citizen should be available to all citizens. This policy would naturally severely limit these kinds of operations without asking us to make broad decisions about delicate matters.
If you insist on defunding something, de-fund and kill the TSA. Then eliminate DHS and have the work go back to the separate agencies. But don't take a flamethrower to something which has a useful, important purpose because the politicians changed their mission to something terrible.