Thankyou! The art direction is definitely the part of the process that I enjoy the most. I'm trying to structure future projects to be even more focused on that kind of stuff; the atmosphere etc..
So that's a pretty big topic but I'll try to be brief. The most important thing is to have a strong vision for what you want to achieve at the start. For me, that means doing sketches and getting a lot of good reference materials. I'm constantly taking screenshots and doing paint-overs on them. I knew Unreal 5 could handle rendering a lot of geo at once so I pushed my style in that kitbash-focused direction.
I'll look at my art target, then what the engine is giving me and try to problem solve my way to get closer to the target through shaders or textures or whatever tools I've got.
A lot of times game engines, (especially Unreal which can be quite heavy on effects by default) are doing a lot of extra junk visually that I don't want or need, so understanding what they're doing and then tweaking those features can be helpful. I always turn off the auto-exposure histogram for example.
You've done some impressive work yourself, if you ever want feedback on something you're working on don't hesitate to reach out.