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Halacha leaves room for more relaxed coverage in more private spaces, and instructs people not to look into others’ private domains. We are also not responsible for others’ choices of where to look. Therefore, even with the sight lines as described, your back garden is fundamentally considered your private domain (akin to the Talmudic chatzer). This leaves room for you to find your own balance between comfort and tzeniut there.
If we understand you correctly, your front garden is fully visible from the street, a public domain. Additionally, the general public tends to feel comfortable looking into a front garden without a partition from street level. (Someone looking in from above is less of a concern in this regard.) For these reasons, the front garden as described is considered a sort of intermediate space, and you should take greater care with tzeniut there than in the back–for example, dressing there much as you would to open the door, including something on your head (though not necessarily full head-covering).
Longer term, we recommend constructing some sort of simple partition to provide you with more privacy, which would allow for dressing more freely. Such a partition would not need to be opaque or completely obstruct your yards from view, but a simple demarcation or visual cue to others at ground level of where your private space begins. Even with a partition, the ultimate choices here will not be a clear cut matter of Halacha, and will depend to a large extent on your sense of tzeniut and social norms.