You could make a rule to only use different layers if there is more than one main sign. A main sign with its additional sign(s) could still be combined into only one OSM node.
But for the monster combinations that @Minh_Nguyen has been showing, combining four or more parking related signs of the same level of importance into one OSM node will likely result in not very intuitive tagging.
more OT
Especially parking restrictions tend to get complicated everywhere. I remember Austria having very verbose additions to parking signs but couldn’t find a photo quickly. Would be interesting how these fit into the current tagging.