RPG

Hah, I'm an unabashed GURPS fanboy; I'm very familiar with the huge variety of modifiers! I actually find it less complicated than 5e, though - sure, there's a lot of options for modifiers; but if you need a modifier you don't know, you just eyeball it and narrate it. Look it up later if you want to check your work; but toss out anything you don't need.

There's an often-missed table describing Task Difficulty Modifiers in the core books (discussed more here: https://gurps.fandom.com/wiki/Task_Difficulty_Modifier) - this is the core of how I run GURPS. I figure out how hard I want a given task to be based on what's been described, narrate the situation, and give them a combined modifier based on what feels right. Climbing over a broken wall? Roll your Climbing skill. At night? Eh, roll at -2. We said it was drizzling earlier? Shift to -4. They're taking their time to go carefully, and risking discovery? Back to -2, and we'll roll to see if anyone comes along while they're climbing. Are those the exact right modifiers? I'unno, you want to crack open the books now, or find out what happens?

But, just to keep from overwhelming a new GM, I'd point them at GURPS Lite first - it should be able to handle most of Frostpunk okay, it's only 30ish pages, and it's free!