On surface level the substance is obviously about expectations and aging in the acting/celebrity culture that treats women as an item who is only useful or valuable if she is young and beautiful- but I think I deeper and still related read could be about cycles of abuse and child stars with parents and an industry that abused them until they break down.
Elizabeth is supposedly washed up, career tossed in the trash once she got "too old" to be a desirable object. She's understandably bitter about this so she takes drastic measures, the substance. (read; either having a daughter or deciding to push her already existing daughter into this career)
She brings Sue into existence and immediately auditions for the very same position she just lost. Willingly putting herself back in this vicious cycle of objectification and abuse. Sue is overworked, constantly clearing her way through the industry. She is expected to be thin and desirable while Elizabeth grows bitter that she is succeeding in the role she herself trained Sue for and out of spite makes messes out of a huge amount of food in a way that is depicted as very grotesque. But the more Sue is pressured into the industry and the more embroiled in it she is she literally takes from Elizabeth to keep going, very literally draining life from her in the form of the spinal fluid. I think this could be taken as Sue's career actually draining Elizabeth of energy and time but it could also be from Elizabeth's tainted perspective, seeing Sue as selfish taking and taking from her even though she's only doing it because of how she was forced into the industry.
In the end Elizabeth tries to end Sue. This could be seen as career sabotage as her jealousy and anger and bitterness builds to its peak. The subsequent event of Sue instead killing her could be seen as breaking ties with a toxic mother. But it's too late either way. In the end Sue falls apart, literally. The culmination of all the stress the abusive industry. It's too late to get out.
But it's not just metaphorical and emotionally a breakdown I think there's been strong and obvious signs of eating disorder from the beginning. Sue is expected to be thin. We never see her eat to my memory, just drink water or diet soda. In one of her nightmares a chicken leg is visible under her skin while she is shooting a tv show and the crew stops and zooms in on it. In the ending sequence her hair and teeth begin to fall out, unhealthy teeth especially being an indication of ED. Yet she doesn't put anything to a stop instead literally transforming into a monster but still going back for the show. Her body image destroyed so that all she can see is that monster. And the industry she has been forced into under no choice of her own ultimately rejecting her when she is no longer desirable.
And on a side note I think the multiple scenes of food being depicted and consumed in a really grotesque way is very intentional. Harvey is an old man who creeps on young women. He is not young but that doesn't matter when you're the rich man on charge. So it's fine when he eats in a very emphatically gross way in public. He's allowed to.
Meanwhile Elizabeth who we're told is old and ugly and undesirable only eats in private, when she makes the huge spread from the french cookbook it's intentionally gross and meant to be seen as such indicating that this is not acceptable behavior for a woman but because she is no longer a useful object she does it. Still in private though. Interesting also that the cookbook was a "gift" from Harvey when she got fired, as if to say "well now you're allowed to go eat whatever you want hidden away in your apartment because it doesn't matter if you stay thin you're too old to ever get another role or be desirable".
And of course again we never see Sue eat, and certainly not in public. Because she is being a woman in the "right" way, not letting people know or see she even eats, staying thin, staying young.
And needless to say this is not a correct depiction of food or saying we the audience should be disgusted at Elizabeth or shaming her it's instead a very purposeful depiction of unhealthy and unfair standards women are supposed to adhere to while the rich men in power do not.
Anyways yeah I've had some thoughts brewing about this movie because people say it's very basic in its messaging and I suppose it is simple and straightforward but there's also a lot more under the surface you can read into.