I wonder if it would make more sense to design the level in the engine or combine them all in blender. I will draw the models from quixel and fab, I am just learning, which would be the cleanest?
Bump as it’d be interesting to see if Nanite has made people think differently about this. But the general wisdom for eons was MODULAR is the way to go. Look at the Marketplace / FAB for example. Packs are usually sold as modular… As it helps with building levels / maps, to be able to tweak things inside the world, especially with adjacent surrounding props / terrain (but also using more accurate lighting). 
Hey there @lortrh32! Welcome to the community! The consensus is that level design itself is just more effective in engine for scenes of sufficient scale and modularity.