You will create a unique game. Experience from other unique games, will not really cover your situation. The question remains if you discover speed problems or design issues first.
Naive implementation can quickly result in lags (e.g. by placing 10.000 objects into the scene) while one large mesh will cause design issues as LOD will not work or you can 't treat parts of the object differently (e.g. animate a tree or open a door).
These are different aspects with different requirements that all needs to be met. - that is what I mean with different shades. You need to find a balance that fits your situation. You already know the extremes.
You want to use LOD, this does not work well then the single object is always near the camera. That is what I mean with - you answered your own question.
sdfgeoff gives you a short and nice “shaded” answer ;). I guess this is what you are looking for.
make the objects in a size that you can manage it.