The land itself should have the same classification as the building, which looks to be an office building, so landuse=commercial. This should include the building, parking & associated soft landscaping (the garden-type features). There’s no harm in having additional landuse polygons (grass, flowerbed) inside (or on-top-of the main landuse category: it is up to consumers to resolve these issues, and although fiddly can be done (I gave a paper on this subject 8(!) years ago at an OSM conference).

I would certainly avoid landuse=grass for non-grass-features because such areas may well be treated as walkable areas. I also see no particular reason to avoid using leisure=garden (you always need an access tag as people use it for private gardens as well as public ones and ones which can be visited for a fee). However, it is not strictly a garden, but garden elements in a slightly different context. Other than the flowerbed we don’t have a consistent way of mapping garden & other soft landscaping elements (flowerbeds, planters, raised beds, shrubberies etc) & we could do with one. Somewhere I have a half finished list of such features. Obviously trees & hedges can be mapped.