2d top-down openworld - How to?

Have you tried loading the whole map? Assuming you will need to chunk the map is what developers often call “pre-emptive optimization” and is usually a bad idea because oftentimes we can spend a ton of time optimizing something that didn’t need it. Or even worse, our “optimizations” actually result in overall slower performance.

Whether you will need to chunk the map and load it dynamically depends on a lot of things in your game and not just the size of the map. I would try to soldier on without it and see how the actual performance is without it. Because of course if you don’t need to do this, that is a lot of effort saved that could go towards furthering the development of the core of your game. You should also read this thread to make sure you are doing simple optimizations to ensure your map isn’t being rendered outside of the game view (if you follow some rules, Phaser will do this automatically and you often don’t need to chunk) and that you are using the most efficient tilemap options.

If you do follow the advice in that thread, load your map and it turns out it is a performance hog, I have seen a pretty nice tutorial on how to load chunks of it dynamically which can be found here.

Best of luck!