I think you would be fine, but a reasonable precaution may be to place some spawn-blocking structures on that patch of plains on the southern side of the river, such as workbenches or campfires.

A single fuling, let alone a pack of three, will obliterate many non-plains mobs with no trouble at all. Because of this they can sometimes aggro-chain mobs until they are quite far away from their native biome. In your case the surrounding water ensures that they only have one way to go, which is in the general direction of your 'high defence moat' marker.

Even if you've cheesed the landscape so that they can't access your high defence moat, putting you or your buildings in danger to their javelins is something that you want to avoid at all cost. And then there are the deathquitos who give much less of a damn about your obstacles.