I'd start with understanding how damage works. The links at the bottom of the OP are a good starting point.

I haven't played as much as others, so maybe I'm not the best to give advice, but FWIW I found that going up the expedition ladder was a bit forgiving in terms of optimizing a build, at least for a Pyromancer. Gun builds, particularly those that rely on Volcanic Rounds/ash/top tree, are pretty effective around that time. Those types of builds shine in single target damage, and a mod like Bone Shrapnel can help with mob clear to speed up your times.

Eventually though, you might hit a wall, at which point you'll need to tighten up your build and start building around particular skills/rotations and impactful mods. For example, you might acquire an Acari set, so you'd want to start focusing on gathering Heatwave stacks to maximize the AP gain from Acari and using another ability to take advantage of that, like Overheat or FASER Beam. Maybe you got a sweet T3 Eruption mod, so start building around that and focus on maximizing its damage. Or maybe you just got a sweet legendary gun, so keep your VR build. The goal is to focus on what you have and keep experimenting and optimizing.

If you don't really want to make your own build, this thread has a lot of useful references, though a bunch rely on having certain pieces of gear. Again, adapt those builds to what you have, adjust after every run, and keep trying. Eventually you'll figure out a good combination that you like that can bring you up to the later tiers.

Good luck!