It is an interesting range of filters, some comments
covariate data: are you specifically looking for gridded formats, such as tiff, ecw, netcdf? Or maybe better to specify by: satelite imagery, soil property distribution predictions, DEM, ...
projects; we currently don't have projects (and persons/organisations) searchable, i can see that being usefull, eg: horizon europe projects, living labs, research consortia
dataset is obvious, but what is knowledge, I prefer to look for knowledge categories, such as journal articles, reports, thesaurii, video's, tutorials,
Soil property / indicator should be findable via its codelist
is using/is citing/is based on relations should be visible via links, such as those provided by crossref.org, although in theory you can also derive usage by evaluating the usage logs (if the server is in our responsibility)
Additional query requests for: