My method is always: messiest --> most detailed work.
So for me it typically goes base coat --> primary color scheme --> washes --> highlighting/dry brushing --> detailing --> free hand work --> touch up. I try to work so that screwing up something on my current layer can't drastically hurt anything on the previous layers.
Thinking of re-doing the model and undercoating him in wild colours and then putting the black over the top so there's more continuity between shapes, which might look less jarring.
I would think black would obliterate most of those color choices by going on second. Black can be hard to give a nuanced look because it's, well, black. It also mutes the saturation of anything going over it, and so takes a lot of the punch out of colors. Going the other way....not many colors can alter the saturation of black unless you do a really, really thin coat.
Black works as long as you have enough sundry detail picked out to break it up. (Also why drybrushing on black models is fairly important to bring out the details.)
If you're going to strip him down and start again, consider base coating in white, and only doing black for the sections of his armor you actually want to be black, then doing the non black sections by hand. That will make the non-black sections pop more if they're going over a base coat of white.
Also just a question: is that like black spray primer then a coat of black paint? or is it just black paint as the primer? I ask because the coat looks a little thick on that Slaanesh marine, and very thick on the guy above.