Can I delete geometry by rendering? I know it could be weird, but can I for example take all the objects in the scene (static objects) and then render them with low polygons? like transforming them or something like that?
You can use the Decimate modifier.

Select your object, and in the Properties panel, click the Modifiers tab; there, click Add Modifier and select Decimate. There are a few modes to select from, which give you a slightly different result, but you'll probably want to use Collapse. This mode as a Ratio slide, which you can use to control how much to decimate your object.