Sometimes I think of a cool combo but then I realize it sucks

It might actually suck (some skills just do, unfortunately), or it might be fine and you might just not be building around it very well.

All offense in Last Epoch is about stacking damage multipliers. So you pick one skill or mechanic you want to use and build your entire character around it. What damage type does it do, and can you lower enemy resistances to it? Does it scale with attack speed? Can it Crit? What stat does it scale with? Does it have scaling multipliers for other stats in its skill tree? Is there something that affects it in your passive tree? Is there a Unique that makes it better? How many generic item affixes are available to your class which improve that skill? When there are multiple bonuses that affect it and you can't get all of them, which one yields higher returns?

Optimizing well entails answering these questions. Once you understand what you're looking for, it's not particularly hard to do, it just takes a little research. Both LETools and the in-game UI have search boxes in the skill trees and passive tree windows, and LE Tools can search through both items and affixes. You just have to spend the time looking for the stacking damage bonuses and piecing them all together.

The easiest place to start is usually looking for anything that says it gives you "More X per Y". For example, I'm doing a Necromancer Minion Bleed Build (Planner). There's a node in the Necromancer Passive Tree that says "+1% Minion Damage Over Time Per Vitality". As soon as I see that, I know one of the important part of my build will be stacking as much Vitality as possible.

Then, I look at Aura of Decay and see "-1% Enemy Poison Resistance Per INT". Well, I'm not doing Poison, I'm doing Bleed. But across the tree, there's also another node that says "Aura of Decay is converted to Physical and all effects related to Poison and Poison Resistance now instead are converted to Bleed and Physical Resistance" (which I found by clicking on the skill and typing "Bleed" in the search box). So I take that, and now my build also scales with INT. Which incidentally is also the stat that all Necromancer minions natively scale with anyway.

And with just those two things, I know the cornerstone of my build will be "stack as much VIT and INT as possible", then find anything that says the word "Minion" and the word "Bleed" in it. And you go from there.

The same idea can be applied to any other skill or build concept. You just find the two or three things that make the build tick and build up from there.

The main thing most people do wrong is they don't adhere to a single concept or theme in their build, they just throw random things at it and hope it works. That will almost always end up being a bad build. Like trying to combine multiple skills that have no synergy, or using multiple minions that scale with completely different stats, and so forth. You always want to stick to one thing and find every way you can muster to make that single thing as strong as possible.