I think the line will always be a little arbitrary.
Clearly on one side of the line I would put basic self-care. People who don't do it, or who don't take any notice at all of social expectations, are often (not always) people who may struggle with mental health. There is something about wanting to be presentable in a basic way that is attached to being mentally healthy.
On the other side of the line I would put things that are invasive and dangerous. Breast augmentation, liposuction, would be examples. The cost of these is also a factor.
That leaves a lot of middle ground. I think an important point though is that beauty standards are never really individual, what becomes normal affects the perceptions of everyone. If most women dye their hair, grey hair starts to be perceived as a sign you are a lot older than you are, that is how people "read" it. That creates pressure, even if it is just internal pressure, when you start to get grey hair in your late 20s or 30s. Or look at pubic hair removal - something we'd think has only private significance, but when I was a young woman living in a university residence I can tell you that girls felt a lot of pressure about it.
So, I think people have an interest in avoiding beauty trends that are very uncomfortable, invasive, time consuming, or expensive.
I would but fillers and botox in the very questionable end of that - they cost a lot and they are invasive and there can be health effects. Even hair dye or public hair removal are undesirable trends in my opinion, they represent a lot of time and money, and the latter can have undesirable physical effects.
But I don't know that i would say they cross a line. Myself, I wear make-up sometimes, I have dyed my hair in the past. But part of the latter in particular relates to feeling like I looked a lot older than I was. We're all effected by the beauty standards we see around us, and maybe it's worth talking about which ones are really negative for women.