What I’ve done is created a temporary place (make a place and then delete it afterwards) that includes our property and then use that place to batch edit observations and obscure all the locations and add them to a traditional project. Using this method none of the locations should be publicly visible as long as you’re careful to obscure every observation. I was initially planning to just use a tag instead of a project (less visible, just as easy to search), but the batch editing got an error trying to add a tag to so many observations and didn’t work. So basically, yes a place/project or a tag could work just as well depending on which you prefer or which works for you.
If you’re less concerned about privacy then just making a place and a collection project for that place would work.
Not yet, feature request here: https://forum.inaturalist.org/t/automate-geoprivacy-with-user-defined-geofences/700/
Yep, they’re not wild. Although if new plants grow from seeds dropped by the cultivated plants then technically you can count them as wild.
I’m not sure if I’m interpreting your post correctly, but if you’re just interested in tracking the cultivated plants on your property as opposed to everything you observed on your property, then you could just do something as simple as bookmarking a search filter for all observations by you that are not wild (&captive=true).