DHS can easily be folded into the FBI and cuts made
TSA is a non-brainer, they don't really provide much security, let airports handle it on a case by case basis
Do they really need a NSA and a CIA?
Space force - just roll it into the air force
Bam - that's probably like 500 billion right there
"As a person who doesn't know anything about this, here are my ideas" is, er, one approach to things, I suppose (as someone else mentioned, you've just given the FBI a fleet of over 2,000 watercraft, which doesn't seem terribly sensible).
The reverse might make a tiny, miniscule amount of sense, but that way it is just silly. FBI is a subunit of DOJ with ~35,000 employees; DHS is an organizational peer to DOJ with ~260,000 employees; the active duty uniformed component of the Coast Guard — one unit of DHS — is around ~45,000 people.
Easily? You just folded the Coast Guard, FEMA, Immigration, and the secret service along with many other agencies under the FBI.
> Space force - just roll it into the air force
Already part of the air force.