How to prevent structures generating below sea level and terrain generating as a box?

I'm trying to add an oasis structure to the desert. The oasis is split into two structures, one for the top layer, and the other that adds depth to the pool. Unfortunately, it can sometimes generate below sea level, and the terrain generates very boxy around it. These two images show what I mean, with one image being flooded.
Without the structure generating, the exact location of the flooded oasis is also attached, it is just a pit. It makes sense to a degree why the oasis spawned low down, but it's not ideal. I'd preferably like it to spawn on more flatter desert terrain, without flooding, and without too much of that boxy appearance.
This is my worldgen/structure file:
{
    "type": "minecraft:jigsaw",
    "biomes": "#rocryn:has_structure/oasis",
    "step": "surface_structures",
    "spawn_overrides": {},
    "terrain_adaptation": "beard_thin",
    "start_pool": "rocryn:oasis/ground",
    "size": 7,
    "start_height": {
        "absolute": 0
    },
    "project_start_to_heightmap": "WORLD_SURFACE_WG",
    "max_distance_from_center": 80,
    "use_expansion_hack": false
}