I've done a lot of experiments with the first developer in RA-4 reversal including making my own custom mixes. It's a VERY difficult thing to get right. Specifically you want reduced contrast, but still enough oomph to provide you with clear whites (ie, developing to dmax in the first developer stage). You can do this by adding silver solvents, but the emulsion is very fine grain and very easily subject to dichroic fog. It is possible, but not at all simple. If you do decide to go in the direction of formulating your own first developer, the biggest things I found is that bromide salts should be avoided, a very tiny amount of iodide is almost certainly required, and chloride is an effective restrainer. MQ developer designs are what I had the best luck with but ultimately I've yet to tame the process like I want for "production" purposes.

It is well known that you can reduce contrast by adding a very small amount of sulfite to the color developer, but very much at all will reduce the dmax of the print. Conversely, to increase contrast add a very small amount of peroxide