Do you have a local weighing clinic? Usually you can go there and see the health visitor in person and ask any quick queries, without needing to have a home visit. They will also often be able to offer weaning advice if you are worried about this. In many areas a course is offered to show you the basics.
Just to say though DS1 was also not sitting up unassisted at 6 months and I remember it being a big topic because BLW was new/a big thing back then and there was all this about them needing to sit unassisted - which was not correct - they only needed to be able to sit with support. Also, a couple of months later he was one of the first NCT babies to crawl. There were two of them and the others didn't start for a few weeks after that. They just choose to work on different things at different times!
Remember that with any groups of babies like NCT, you'll notice all the things that the other babies are doing which yours isn't yet because it will stand out as odd, but you won't register the things that they aren't doing (but your baby is) because of course no baby does them all the time so you'll just assume that they are doing those things at other times :) NCT is also a small group, so you get odd clusters which aren't actually representative of the wider picture.
Agree also that rolling sounds right for her age. The red flag to watch out for would be no rolling at all by 6 months. You have already passed this so not to worry if she doesn't roll both ways yet.
In terms of sitting the milestone to look out for is that they can support their head e.g. if they are placed in a sitting position in your lap or against some cushions or some kind of baby seat. If they are still floppy by 8 months old, this is a red flag and you should seek advice.
Most of the milestones they do before 4 months are easier ones and so they happen pretty fast. After 4 months you're getting to the harder milestones so they take a lot longer to work on. Perhaps this is why it feels like things have slowed down? Don't worry.