Matching properties can be scrubbed simultaneously across multiple selected layers. Select the layers, hit S to reveal the scale property, select all of them, scrub. Of course you need to find a suitable reference value and eyeball it a bit. Otehr than that applying a simple value*2.803 to all scale properties would give an exact scale.
Increase your comp size to that ~5000x3000, create a new Null, parent all 30 layers to Null and then increase Null's Scale. You can even replace your low res. background with the high res. one so as to monitor if the new scale looks nice.
Copy and paste this expression to one of the layers, then, select scale and select Edit>Copy Expression Only, then select all of your layers and Paste....
value * 2.083
That's it. EZ as pie... This will multiply the existing scale of all layers by 2.085