If you don't get it work, you might try to move all the CSS/JS postprocessing into a dedicated build step and use the frontend build process of the newer archetypes (see  for the ui.frontend module created by archetype 21). Depending on your frontend developers they might prefer that way over the "classic approach" 🙂
Actually the "new" approach isn't new at all. It just embeds a node-based frontend development process and allows to use maven to run node/npm as well.
Thus your FE devs can work within the ui.frontend module and use their tooling. And if you build everything through a jenkins or any other buildserver, you can use maven, and the FE pipeline is invoked as well.
But if you already use NPM, why don't you do the minifaction there already and disable any postprocessing within AEM?