Hi Geeken,
That’s a really broad and multifaceted questions. But I can respond in the context of what I’m seeing in the image you posted.
A black void behind the buildings. - I don’t think there is a single other person on this forum who has as deep of an appreciation of the Void as I do. Darkness within darkness. Timeless, perfect, all things are born from it, all things return to it. So I’d say double-down on the Void Vibes by disabling the time of day manager in Window → World Settings or add something (landscape?) that matches the vibes you’re trying to create in this map.
I’m also seeing a lot of prefabs, this bores me. I’d modify the prefabs or start making your own buildings with the modular prefab kits.
Also, improvement might come in the form of throwing away what you’ve built and starting fresh with your newfound knowledge and tastes.