Perhaps you could start by fleshing out just one neighboring culture - one which would have a lot more interaction with the region the players are in than others.
That could give you some idea of how much time and effort is involved, and help you figure out where to scale back for just a rough sketch of the others.
I don't think it would be possible or practical to try and "invent" these other cultures whole-cloth, and expect them to be on par with your Europe-analagous area.
So, it would make sense to make the other cultures analagous to other real life cultural regions. Thus, your question is - how to treat them without resorting to ugly stereotypes.
Perhaps you could study sources from their own cultures, and see how they themselves write about their cultures.
We've come a long way in terms of more sophisticated variant concepts from Orion ... mini-mag Orion ... anti-matter catalyzed micro fission ... all sorts of ideas. Heck, there are even non-nuclear variants using kinetic impact powered explosions rather than nuclear powered.
Ironically a lot of these advances in ideas come from fusion research, rather than fission research. Fusion research gives us experimental data on magnetic plasma containment, plasma compression etc...
So, even when, say, a Z-pinch experimental facility is unable to produce conditions suitable for practical fusion power, it still provides data on a way to compress and heat plasma to extreme temperatures.
That's data that someone can take and use to design a plasma thruster. No need to achieve fusion, just need to heat it up and let it escape out a nozzle.