Questions about minimizing draw calls and rendering system design in generally

Hi. I’m currently in developing of 2D/3D indie game (isometric game with 3d character models and 2d level) on my own engine and now I’m thinking about minimizing draw calls.
For level design we’re using level editor called Tiled. The average level consists of 6-20 layers each represented a part of floor, roof and so on.
When i’m loading the level, my engine creating tileset atlas, (which consists of level tileset images glued together), then i build a vertex mesh for each layer (each tile represented by 4 vertices, each vertex contain texture coordinate of tileset atlas). Some of the 2d stuff (eg walls) also have a peculiar normal map (which stores in another texture atlas), which uses to give player the impression of a volume when this object is enlighted by light source.

Thus, i need also the normal map atlas (consists of minor normal map images - one for each regular tileset). All the 2d objects in engine have the same vetex data type and stores in one VBO (even the ones without the normal maps - for such sprites the appropriate place in normal map atlas left transparent). All the interactive/movable 2d objects which are not the part of level geometry are also in this VBO. All the 2d stuff uses one shader, wherein through the series of if…else statements decided how exactly current primitive should be rendered (i use QUADs primitive for 2d objects). Positions in a world space of each 2d object are set as uniform variable.

3d models are animated (which means that i need to set animation matrices in shader as uniforms), texture of each models are places in the similar texture atlas while level is loading (there is around 10 different model textures per level). All of the models stored in one big VBO and uses one shader, that works similarly to my 2d shader - inside shader is a bunch of if…else statements as a result of which (and, also, the material of a model) it is decided how to exactly draw an object.

I store all of the mentioned texture atlases in the GPU texture memory all at once so i don’t need to change texture units when drawing different models or load other textures to GPU memory.

So, i have several questions:

  1. First off, is my rendering system design any good?
  2. Second, sometimes i have multiple characters, which uses the same mesh. Is it better to store several identical buffers in VBO, or draw one buffer multiple times (i heard about instancing and i don’t want to use it, because i regulary have from 2 to 10 identical objects and not hundreds of it)
  3. Currently i am drawing each object (i.e. one 3d model/level layer/2d sprite) by a single draw call, but i want to minimize draw calls - best case scenario is to have only two draw calls per frame - one for drawing 2d stuff and one for 3d. Best idea i come up with is to pass to a shader two uniform arrays of object positions and orientations for each object and build transformation matrices directly in shader, but in this case i don’t know how to identify an appropriate object in the shader (except, maybe, having an integer value in the vertex data struct of the meshes, which represented an index of the current object in this uniform array - but this means having a lot of repeating data pieces in VBO).
    Other idea is to have an additional VBO with positions of each object, but in this case i need to have VBO entries for EVERY vertex i render, so, again, it means (without instancing) a hell of a data repeating.
  4. Maybe i just worry too much about draw calls? Usually i have around 30-70 objects per screen, not thousands. Maybe i should let it be as it is - with a single draw call per object?

Please, give me some advice.