I am using this para- Amino Phenol type developer since 1967 and control the temperature always between 18C - 20C. Equal processing temperatures during all steps were in the past very important and still is for classical type films like Efke. By reducing the agitation you can reach reasonable fine grain with Rodinal.

But for extreme grain reduction you need an Ultra Fine Grain developer based on Phenylene Diamine (Sease III, Windisch W665, MCM100). But you will loose at least one F stop with Ultra Fine Grain developers and not all type films will react in the right way.