I think that around 18 mo is quite a common age for LOs to get "pickier", so sounds quite normal to me in that regard. Sometimes it is related to teething canines as that can put some LOs off their food a bit (or a lot!).
ITA though that you just keep offering healthy options and let your LO decide which and how much they want to eat.
Sometimes my DD has been more tempted to try new things if it's in a bowl on the table from which people can help themselves, rather than me putting it on her plate when I serve. Perhaps you could try that?
Also, DD is often much happier to try new things when I am preparing (e.g. chopping) them rather than when they are at the table. So perhaps you could offer her some tastes of things as you are preparing meals? You could also get her involved in simple meal prep, e.g. washing veg is an easy job for them to help with at this age.
I agree with PP re mealtimes, just keep modeling. But it will take a while, especially as she's still so young. But I'd also perhaps decide what would be the minimum you'd expect and don't expect too much iyswim. You say she finds it difficult to sit still for long. Is she in a high chair? If so, could you bring her more to the table? e.g. by taking the tray off, or getting her a booster seat? Would you be happy for her to get down when she's finished (i.e. had enough) but before others? You mentioned other LOs. If some of them take a while to eat then perhaps it is too long for your DD to wait at this age...? I'm not sure if she's quite ready for this, but perhaps when she's a little older, could you find her some jobs to do which involved fetching/taking things so she gets to move about but is still participating in mealtime?