If NASA built their manned space program around the SpaceX Falcon Heavy and Blue Origin New Glenn (and future uprated versions of both), they could start launching crewed vehicles into deep space next year at less than 1/10 the cost of the SLS that won't be launching humans for at least 4 years.
It's not just that they have been held hostage by congress to build the SLS as a pork delivery service. They've also become risk averse. The Saturn V was built with "all up" testing, rushed to testing a completed rocket instead of focusing on component testing. They only flew two Saturn V unmanned missions before they launched one with men on it. Today, SpaceX has launched the Falcon 9 over 20 times, and has a capsule with the safest abort mode ever, and NASA still hasn't man-rated it.
NASA could take a fraction of the money they are spending on the SLS, and start doing monthly deep space launches by the end of next year. They could use 140,000 lb capacity Falcon Heavies and Dragon Capsules to do lunar missions. They could put astronauts back on the moon, build a constantly manned moon base, develop and test rovers and other equipment they want to use on mars.
Astronauts would be lined up to volunteer, even if the Falcon Heavy only has unmanned two test flights. They are far more rational judges of what the safety levels should be than the PR department at NASA.
Then within a few more years, NASA could shift to doing Mars missions when SpaceX and Blue Origin or anyone else can start giving them 300,000+ lb cargo capacity launches for less than $1,000/lb. At that price again they could average a dozen or two dozen launches a year. All that launch capacity would enable them to launch a group of Aldrin Cyclers to provide regular transport to mars and back with heavy radiation shielding, supply storage and room for big crews. Other robot launches can pre-cache supplies, equipment and return fuel on Mars.
But they can never do it using the SLS path. It's going to start off costing near $20,000 per lb for LEO access, and even the later versions will still cost over $10,000 per lb. That just makes Mars missions almost economically impossible. The SLS could only do an Apollo style program, where a decade from now they launch a handful of all-in-one missions (two orbiters, a couple that land) before congress wilts under the enormous costs.