Worldbuilding a Gunpowder Fantasy Setting

"Gunpowder weapons?! In a fantasy game?! Inconceivable!!"  - too many people I've previously played with.
It's finally time for me to actually write down the concepts that have been floating around in my brain for the past several months. It's going to be the setting I have in mind for an (overly?) ambitious solo RPG I'm writing. The working title is going to be Powderborn for now.

A human-centric fantasy setting with technology comparable to the 18th century.
Flintlocks, early rifles, artillery, cultural revolution, age of sail-style ships, political experimentation, global trade, and magic that is still very new and not at all understood.
How would early industrialization play out when it comes hand in hand with the discovery of magic?

Not a generic Medieval-fantasy-but-actually-Renaissance-no-wait-floating-steampunk-ships-too + guns setting but a dedicated world, built from the ground up for a very specific type of game.

The most popular games often come with a kitchen-sink setting that tries to cater to every group and every possible style of play. This works perfectly fine as long as you don't think too hard about the cohesion of different parts and how the world doesn't implode every other day.

Over the years, verisimilitude has become very important to me in works of fiction. Every time I hear "Ten thousand years ago..." and the technology hasn't changed (without reason) one bit in all these years, my eyes want to roll into the back of my head. (Lord of the Rings feels like mythology, so it's not the same.)

The Core Ideas

• Human-centric
• 95+% of typical inhabitants are humans (excluding flora and fauna)
  
• No 'gods walking among mere mortals' (high-level characters)
  
• Intended for skill-based systems (no levels)
• Magic is still new to the setting (~80 years)
• Some of the first human magic users are still alive
• Magic is dangerous, and every spell comes with a price
• Energy output requires equivalent energy input
• Outsiders arrived suddenly and violently (~80 years ago)
• Known as the Rheokrene because it was the source of magic.
• Classified as invaders, explorers, refugees, settlers, etc. (depended on the location)
• The Rheokrene drastically changed the geology, climate, flora, fauna, and other parameters of the world
• Mountains disappeared, lakes dried out, jungles grew in deserts, cities appeared out of nowhere, etc.
• Animals mutated, plants evolved rapidly, and it appeared as if the laws of nature were altogether broken
  
• This allows for a ton of variety in gameplay, while still staying true to the established rules of the setting
  
• Magic isn't fully understood or solved
• The outsiders slowly taught their magic to some humans
  
• Many different interpretations and approaches to magic
  
• Magic is affecting everything in the setting
• No 'this is a European-style town and oh btw some people can also cast spells'
• How does the world not constantly blow up with the destructive power of magic?
• How does society function when illusions, enchantments, etc. are possible?
• What are the countermeasures?
• Cultural and political upheaval (early signs similar to this)
• Drastic shift in power structures
  
• Monarchies and Empires are crumbling, what replaces them?
• Religions in decline
  
• Disillusioned former believers feel betrayed and abandoned by their god(s) because they did nothing before, during, or after the Rheokrene
• Magic is real and allows humanity to unlock its full potential
• Why believe in gods that do nothing, when humans can do more powerful deeds on the regular?
• Rich history before the introduction of magic
• How did civilizations develop across the globe? How did cultures evolve and intermingle?
  
• How did the Rheokrene affect the ruins and remnants of past civilizations?
• What about the new locations that appeared during the Rheokrene?

The Next Steps

Writing down the cornerstones of a setting is always helpful, but where do I actually start now? There are many different philosophies when it comes to worldbuilding, and it is far beyond the scope of this post to discuss all of them.

Usually, I prefer the bottom-up approach, where you start small with a little village (for example) and then slowly work your way outwards and expand the settings as you need it. This works perfectly for multiplayer games and for solo sessions where you want to be surprised by the dice.

For this setting, I'm going to have to use other techniques because I want to focus on a cohesive setting with believable internal logic. That's hard to do if you only focus on one part at a time, and would likely involve a lot of time-intensive retconning.

All that being said, here is my plan for the next step:

Do NOT start with a detailed map (a sketch is enough)

Many people (including me) have started worldbuilding projects by carefully crafting or generating a map of the world. This leads to quick gratification and an illusion of progress. Sure, the map may look pretty and full of names, but what's the point if there is no substance behind it? You only limit yourself by drawing hard borders and landmasses that will most definitely restrict you further down the line.

A rough sketch is all I'm going to do. A landmass here, maybe an island here, and another big continent over there. The Rheokrene allows me to ignore Earth-like realism in terms of geography, so I'm just going to go with my gut. Plate tectonics, climate patterns, biomes, etc. are all fascinating topics that are in no way needed for my current goals.

I want a rough overview of where the major powers might be located and how the landmasses might affect the civilizations and cultures living on them.

No concrete plans yet for how I'm going to share this worldbuilding project, but I'm definitely going to check in for major milestones.