In general, starting out, probably safest to keep individual things separated. You will have greatest flexibility this way. In programming they have the Single Responsibility Rule and even in art world I think the principle is useful to follow (at least during production when you are figuring things out.)
A thousand times throughout your project you will change your mind about what you need, don’t need, and best way to organize. If you keep individual things separate, life will be easier through this process.
At the end, you have many options for how to combine, batch, occlude, etc etc all the final art that has made it to the end. On Youtube Unreal channel has a few in-depth videos about optimization techniques, but the general idea is always simple: combine like things, remove things that aren’t seen.
TL:DR - don’t worry about optimization until later. Try to keep your art workflow as flexible as possible so that you can continually experiment. Optimization is extra work, but all of your art is highly variable for a long time.