Sandpaper is much too agressive. You risk paint damage on the sharper edges as well as a directional stroke look which never seems to look right.

I've never heard of a "de-glosser" but if you can get some and it works out that would likely be the way to go.

Otherwise the trick would be to varnish the existing paintjob with some matt or flat good quality polyurethane varnish to add a non glossy clear coat.