When adding rings, make use of the advanced menu (the pen symbol on the selected ring type).
Select ordered disc and open the advanced settings.
Number of Bodies: 100 (experiment with that number for performance)
Bodies instead of Particles: ON
Mass=Parent/Count: ON
Color Mode: Manual (matter of taste)
Material: Silicate (other materials in other rings, to be added later)
Inner Radius: 0 (for other materials 0 to 3, try around for best results and fewest unused debris)
Outer Radius: 100 (for Silicate)
Radius Units: Radius
Motion: Auto Orbit
Velocity: up to experimentation (since the auto orbit will get lost pretty soon after unpausing with the center planet deleted)
The inner and outer radius define the range at which your materials will spawn, while the numerical range will define how concentrated the materials are. (higher concentrations = higher likeliness of collisions and "bodybuilding", but also less likeliness of materials outside that range being used)
So far I got the best result with silicate spread across the whole width of the disc, iron at around 0-30 radii, water at 1-50, CO2 at 1-70 and organics at 0-100 again.
The sim will run considerably slow at first but speed will increase after enough collisions reduced the number of bodies. If you get too many small fragments that don't contribute to accretion, use the advanced powers menu to delete all fragments. But don't do that too soon, leave them a chance to return and hit the bodies that build up.
Once the sim runs fast enough to decide that too many bodies are escaping the accretion area you can use the "delete escaping bodies" power.
For performance reasons, disable labels and trails/orbits and everything that's not really needed.