It's a shortcoming of spritesheets that you can only load entire spritesheets in to memory. In the extreme case there would be all content in a single sprite sheet, and you therefore load all content in to memory at once. However spritesheets themselves save memory by packing multiple images together, and they tend to be fragmented over a set of different spritesheets anyway. For example one of the demo games we test with is very large, and still uses 100 sprite sheets, which allows for quite a lot of granularity around what is loaded.
TBH unless your player animations are absolutely huge with loads of frames, splitting things out in to separate objects is probably a micro-optimisation that doesn't make much difference. It also can jank the game as it loads textures mid-frame if the object isn't already preloaded by being placed on the layout. And currently the engine doesn't release textures until the end of the layout, so the peak memory usage will be the same.
I don't think the spritesheet system will cause any memory issues, but if there are cases where it causes a problem then we can look in to figuring out how to address that. If we had to do that, I'd try to avoid any kind of manual configuration, the groupings can probably be determined by which objects are used together on layouts.