I’m 34 already and just starting.
I’m 35 in December, trying to change career from academia. It’s a mountain. I’ve been using Blender since February (if you want hard mode, try doing it jobless in a country where you aren’t fluent in the language!). Anyway, another kindred spirit perhaps!
As far as tying things together goes - well, I’ve found a limited colour palette can help. Modelling things to real life scale also helps. Another potential tip is to just model everything as accurately as you can - then you don’t need to worry about consistent detail, because everything has “all” of the detail. This will make you end up with scenes with millions of polygons however…(unless you can do high poly to low poly bakes).
I think light is more important than most things for tying the scene together. I mean, I look outside my window now and Kaunas is a riot of different coloured and style houses, some greenspace, etc. There’s no consistent 5-colour palette! But the sunlight, distance haze, refraction, etc., make it a “scene”.
Also, as you can see here, compositing really helps too. You might see in some of my work that things are sometimes not quite “seated” in the scene, and I think if I’d known what I was doing with compositing a lot of those issues could have been lessened. Very helpfully a bunch of folk better than I have just commented on how they do it…
For textures - can’t say much on substance as I have never used it. But there’s a lot of free or cheap textures out there (+ procedural), you can get a lot of mileage out of them. I think if you already struggle to link scenes together, having access to infinite self made textures from substance is probably not going to help.