All sounds very familiar. Agree to see if you can access some dietician support, they have good advice and can prescribe supplements if needed. There are some ARFID FB groups, but tbh the communities around this diagnosis are much more focused on the extreme restrictive diet type of ARFID vs just not eating enough.
Based on what we’ve found useful, that is just about keeping us on track:
Assume you already know the need to eat 5-6 times a day, smaller meals but more often. Puddings after every meal, forget usual healthy eating rules.
Its much easier to drink more calories than eat them, and although milkshakes and smoothies are best, you can get fortified juice. I do a fair bit of baking now as I can boost calories by adding ground nuts to flapjacks for example. Adding butter, oil and cream to anything I can. If she’s off gold milk you could try adding powdered milk to normal milk, it doesn’t change the taste to the same extent.
Also have a look at satiety index information - some food gets digested much more quickly and some makes you feel fuller for longer (boiled potatoes the worst for this). If you can focus on low satiety foods, she should feel able to eat more a couple of hours later. Fast digestible carbs are low satiety, e.g. brioche or croissants.
Vitamin supplementation.
And try to keep pressure low. Gentle prompting to have a little more, fine. Bribery, more direct pressure, impatience and frustration (god it’s hard though!), talk about calories etc, not fine. I’ve had very clear steer on this from the clinical psychologist we saw.
I recognise the slow eating issue and talking too much. Although distraction while eating can sometimes be helpful, I’ve found it counter-productive because it leads to forgetting to take another mouthful or even chew and swallow what’s in their mouth. Eating together to role model pace has been more helpful for us.