I would say look at trail boss. I shot some cast with it and it was fairly slow.

That's the best I've seen. Hodgdon load data says a 440gr cast bullet, 2.025" OAL, 7gr trailboss is 643 fps, 14,600 psi.

That figures. I don't deal with trail boss or severely reduced loads myself other than the occasional fun. The other old way of making cat sneeze loads is to just use a fast flake handgun powder such as bullseye, 700x, red dot, etc. You aren't going to over pressure a 500 S&W at 500-600 fps. Definitely start extremely low, like 2 grains of powder, and do this over a chronograph plus make sure the bullet actually exits the barrel every shot. You won't find load data for this. Also I would for sure use only cast bullets for this.

As others have said, Trail Boss is both the king of this and also unobtainable.

I worked up some powder puff loads with Promo, too.

Use a chrono, do ladder loads.

Linky

Ramshot Powders has a small section on reduced loads for the 500 SW.

https://hodgdonpowderco.com/wp-content/uploads/2023/04/WesternPowdersHandloadingGuide8.0_WEB-1.pdf

I use 5 to 7 grains of unique behind 250 grain lee REAL bullets and 350 grain sierra jacketed bullets out of my Handi Rifle. More like shooting a mortar at any distance over 100 yards, but quiet and easy on the shoulder.  I have gone as high as 7 to 10 grains of unique, without any issues.  My main interest was using the 50 caliber bullets used in the war of northern agression to see how they would shoot out of the rifle. I found I got a .50 inch group every time I pulled the trigger.