I have a S&W 657 too. After several years of thousands of rounds of cycling/shooting, it has smoothed out.
A "home made" trigger job on these stainless steel S&W guns is possible, but not recommended for the amatuer smith. The hammer and triggers of S&W stainless revolvers are hard chromium plated carbon steel, and subject to the same working precautions as the case-hardened unplated carbon steel parts. That is, the sear surfaces are case hardened, and improper trigger stoning will wear through the case hardening and ruin the parts, making the gun unsafe.
As for making the trigger action lighter, simply replacing the trigger rebound spring, lightening the hammer spring, cleaning and relubricating with moly grease should make a big difference in trigger pull with no other changes. Leave the fancier trigger work to a fully QUALIFIED smith.
HTH
John