Op, yes I completely understand, I was more pointing out that so much marketing gets presented as sustainable when that's not a protected term so it can mean anything. I get quite annoyed about it as there's a lot of 'women and their silly clothes are the cause of climate change' shaming and these marketing strategies play into that by presenting buying something as sustainable as automatically good that's worth paying a premium for. Through your purchase you somehow absolve yourself of this guilt when that is rarely the case, it's just a marketed incentive to buy X over Y.
There's nothing sustainable about the amount of water required to maintain a football pitch or the exhaust fumes at the Formula One but there's no shaming of the largely male fan base for enjoying that. For clothes we can at least claim that they're necessary...
Reality is as an individual it's almost impossible to make the right choice as there are so many factors and it's also financially inaccessible for most. I thought the new clothing rental initiatives were great but on closer inspection it turns out they're more environmentally unfriendly than buying something and trashing it after one use 
when I shop secondhand it's almost exclusively online as there's not much on offer locally. I'm sure if I looked into it deeper that isn't as environmentally friendly as apps like Vinted and Vestiaire Collective make it out to be. There's nothing all that conscious about the H&M conscious line, recycled polyester is just new polyester with a bit of recycled mixed in, recycled ocean plastic isn't actually harvested from the ocean, etc. etc. It's a minefield.
I think you make the best choices you can and this will evolve over time fed by what you come to know, mistakes will happen, it's not the end of the world. But it's a difficult path as trust is a diluted concept in sustainability claims. There's a whole economy surrounding it, a lot of the apps and websites evaluating sustainability or clean beauty claims of companies require payment (from the brands) for positive assessments, etc.
Also, there's limits, I don't ever see myself buying underwear secondhand, but I also don't see myself buying some special technology ones that never requires washing even though that would be great for the environment. I also enjoy new things now and then and there should be allowance for that because an either or setup is just doomed for failure.
There's a German e-tailer/marketplace (a bit like Wolf & Badger) called Staiy, they gather a lot of different brands and evaluate them on different aspects of sustainability, as per a pp it's not always the case that good worker's rights equal environmentally friendly and vice versa but it's also how much energy did it require to produce, etc. They're quite strict about so it's helpful that someone else has done the due diligence and even though there's incentive for them to present things rosier than they are, they feel quite even handed about it. I haven't looked into it closely but there might be some UK based brands on there too.