For residential gardens, you might find that leisure=garden + garden:type=residential + access=private works better than landuse=grass.

If you're going to micromap planted areas in private gardens, landuse=flowerbed (rendered in Carto) or natural=shrubbery (not currently rendered) are better than natural=scrub.

https://wiki.openstreetmap.org/wiki/Tag:leisure%3Dgarden
https://wiki.openstreetmap.org/wiki/Tag:landuse%3Dflowerbed
https://wiki.openstreetmap.org/wiki/Tag:natural%3Dshrubbery