I do thumbnails when I’m being thorough and not rushing, but as I mentioned in my reply to Ramon:
So - I started to make thumbnails, but was impatient and decided I already knew what I wanted. I took that one very loose thumbnail and used it as a base to build the image on. I did a base sketch in greyscale (in values rather than lines), then once I’d had enough of that I started to colourise it and continued in colour from that point (the majority of the work time).
As for resolving those details in my thumbnails - I never take them that far. Mine are a combination of ideation, basic composition and value study. You can see some examples in the sketchbook I haven’t updated in ages: Myth's Tome of Inane Scrawlings 📖
I’ve long wished I was better at doing lineart so I could solve all the composition and drawing problems before painting. But I struggle to draw with only line - I’m more comfortable working with values and never made enough of an effort to change that.
Come to think of it - I did have some success experimenting with adding lines as a layer over my early values, but I seem to keep forgetting about it! 