You could look at a referral to a sleep clinic. There would still be a waiting list, but in some areas it is a shorter waiting list.
I have a teen DS who struggles with nighttime. For many years, he slept with us. He does now start the night in his own bed.
If you want to persevere with DD in her own room, leaving for 5 mins is too long to start with. Can you try literally popping outside to put something just outside the room? Then 30secs, 45secs, 1min.
Can DD discuss what she finds difficult about nighttime during the day? DS1 has poor insight and can't put the ideas we discuss into practice, so talking during the day about nighttime didn’t help. He can't 'do' visualisation either, but if DD can, some find that helpful.
Other things we tried with DS1 included various lighting, dream pad pillow, ear plugs, a 2 way monitor, brushing, worry book, tents, doodle book/pillowcase, music, white noise, various apps, weighted blanket, audiobooks, life size teddy, relaxation. Going against all sleep hygiene advice as a last resort, I tried letting him watch the Lego Movie in bed, but it didn't work. An adapted version of gradual retreat. They didn’t work for us, but something might work for you.
For us, medication helps. He takes melatonin but on its own it isn’t so helpful, but he takes other medication to aid sleep as well.
Some people use antihistamines, most commonly, Promethazine. You can buy it OTC. Although don’t tell them you are using it for sleep.
Is DD receiving any support for her anxiety?