Follow Your Yard’s Evolution
Your yard is not stuck in a time warp; it is constantly evolving. Trees grow and create more shade… and also dry out the soil. The mulch you put in 2 years decomposes slowly and makes the soil richer and looser. The new pool your neighbors put in right next to the fence makes that part of your yard much wetter, etc.
The easiest thing to do is not to try to combat these changes (bringing your neighbors to court for “loss of use” of your property is not a good idea), but to follow them. Move the peony that no longer blooms because it’s now in the shade to a sunnier spot, remove the yarrow that prefers poor, dry soil and now collapses because the soil is richer and replace it with astilbes that wil love the richer, moister conditions, put in plants that like boggy conditions near the now soggy property line, etc. When conditions change, the plants have to change: learn to accept this fact and gardening will be less of a chore.