I just found the answer to my question searching the forum. I need to use the limit validation found here:

Validating field data

Is there a way to control the display of the image once it’s inserted? It looks like it’s using the thumbnail size, which is good for small images, but not for banners.

I don’t like the idea of having to copy and paste fields if I can just reuse them with a function. So the function approach I think will work just fine. I will need to find a way to always create a unique field slug each time, but that shouldn’t be too much of an issue.
I can see this taking a lot longer to setup at first, compared to ACF. However I can also see a bit more flexibility.