various NCT friends say that their same age babies go down for a longish stretch from about 8pm.
That's certainly true. How quickly babies develop is entirely unique and covers a huge scale. My eldest child was walking at 7 months old. My best friends child was 18 months. I would therefore not have expected my friends daughter to be climbing stairs at 12 months old, she developed at a different rate.
Exactly the same is true of babies. One of my four children (DC3) was sleeping 7pm to 7am without waking from 7 weeks old. DC1 was 2y6m before she managed this, DC2 6 months and DC4 10 months.
Just because other babies of the same age can do something is no relevance to your child. They will be at very different developmental phases, and may be for a long time to come. The spectrum of "normal" is vast.
If we abandon the attempt at an earlier bedtime...
I wasn't suggesting you abandon bedtime? I suggest that you establish a bedtime at 7-8pm ish (whenever baby is awake at a time that works for you) when you do bath and PJs and whatnot. But that baby sleeps (naps) downstairs after this bedtime, until you go to bed.
You are aware of SIDS recommendations that baby sleeps in the same room you are, until 6 months old? This includes daytime naps and during the evening.
It's kind of set in stone and well established as NHS policy to reduce infant deaths.
...do you think it's worth instead working on getting her to settle to sleep in cot rather than in my arms
Definately. Well I would. Not the cot for daytime naps though.
The most effective place for daytime napping is bouncy chair in my experience. Sat on sofa, foot bouncing baby to sleep.