Use Falcon Heavy by SpaceX or New Glenn by Blue Origin for a fraction of the cost of SLS.
Orion may be worth keeping. It is limited to 4 astronauts which is probably too small a crew for a 2.5 year Mars mission. I do think that it could be built much cheaper, however.
A Falcon Heavy has close to the capacity of the first SLS version, and about half the later versions. But you can launch 20 Falcon Heavies for the cost of one SLS.
Orion may be worth keeping. It is limited to 4 astronauts which is probably too small a crew for a 2.5 year Mars mission. I do think that it could be built much cheaper, however.