If modular is too much, perhaps just make the pieces bigger! I’m sure you’ve played some StarCraft: Brood War back in the day, am I right? Some design do look inspired from there!
Focus on big pieces and tiny ones instead of middle-sized ones! So GMs can just drop a big block and then add signs, doors and the like at his own pace! 
If I had a central location that I intended to use on many occasions, I would use 3D terrain. For example, the above assets would make a great town that might serve as the PC’s home base. In that regard, I might spend extra time making that map.
The drag and drop aspect is great, but scale works on large and small sizes. The real issue is seeing isometric art in games, attempting that on the tabletop, and knowing how much is going on behind the scenes to make it work in video games.
When I was in school for game design we studied isometric art and why it’s a chore to implement, so my solution for the tabletop was to abstract interacting with assets/space when it came to actually playing with the assets. It works easy enough if you design around it, but selling everyone on those design decisions can be tough if there isn’t a constant standard to measure the abstraction against.
Visually, video games make it look easy with things like layer detection and such, but on the tabletop it doesn’t translate well as you begin to design empty spaces for encounters or lopsided arenas to justify the perspective. In every design you make a sacrifice, but readability and literalism is a rough one to get around.
As a diorama, isometric rocks, and I’d love mock up fantasy cities with it. It would feel like an old final fantasy game. 