I dont know whether this is actually something that works in practice, but would not a matte sealant (to protect the paint) followed by a thin layer of gloss work?
I really like the colour scheme and the effect you created with your paint

thinking about it, you might struggle to get a thin varnish on it if you are painting by hand, I was assuming you'd be using an airbrush, doh.
Could you do it the other way around? a varnish to protect then a thin spray of a dullcoat to slightly reduce the shine? hmm