I'm a community paediatrician and look after lots of children with restricted eating.
Sometimes, if you carry on using all the strategies that you have been told to use, things might improve or you might do everything right for years and it not improve. It's very hard to make much headway with it to be honest.
Therefore pragmatically, I always tell patients that if they are growing, they are getting enough calories. Try and get a multivitamin in her in some format to avoid vitamin deficiency and then at least they are safe. The truth is, you cannot make them eat.
She's also young, so lots of the psychology driven services don't accept referrals until they are 8 so this limits things.
For the kids I see the dieticians will run a restricted eating workshop and give advice about supplements etc, psychology will give some advice about restricted eating and when they turn 8, I can refer to camhs, who will tell them the same stuff again. If they are really bad, I can send them to a tertiary centre.... Who will tell them the same again.
Have a look at the beat eating disorders website as there is also some support available through there.
Personally, I also have an ASD restricted eater at home (although not as limited as arfid). It's frustrating and difficult and I constantly worry about the amount of pringles she eats ....... But her BMI is on the 3rd centile and she has to eat something!
Sorry, the above isn't very helpful. There is a good book I recommend called "helping your child with extreme picky eating"