I don’t think level 10 is going to work. An example i looked up in osm is for instance the 1 year old city of Beekdaelen. This is now level 8 and the old towns: Schinnen/Nuth/Onderbanken are now changed to level 10. These 3 are nicely visible.
BUT
Some years ago city Landgraaf was made out of Schaesberg/Nieuwenhagen/Ubach over Worms.
And looking up one of those 3 doesn’t give me a administrative level 10 at all!
so going for level 10 would give quite some inconsistencies, and although i like seeing more cities completed (in 2019 Beekdaelen was created, and my 3 completed towns were merged automatically to 1 
), i guess level 8 is the way to go.