One thing you probably need to figure out first: how are the dgpu and igpu connected to each other, and then which ports are connected to which gpu.
Everyone does funky shit with this, and you’ll sometimes have dgpus that require the igpu to do anything, or cases where the internal panel is only hooked up to the igpu (or only the dgpu), and the hdmi and display port and so on can be any damn thing.
So uh, before you get too deep in planning what gets which gpu, you probably need to see if the outputs you need support what you want to do.
I’ll admit to having no opinion on windowing systems.
If the distro ships with X, I use X, and if it ships with Wayland, I use Wayland.
I’d honestly probably not be able tell you which systems I’ve been using use one or the other, and that’s a good thing: if you can’t tell, then it probably doesn’t matter anymore.