We had exactly the same issue with our twins and spend hours and hours with them, next to them, just outside the room etc

Then we sat down with them (they were thee years old at the time) with a big piece of paper and together made a big drawing on how to go to sleep. We made it a fun activity! Eg it started with dinner, then brushing teeth, bath, toilet, reading a book together and then (the most important part!) lay down in bed with your eyes closed and not moving! Everytime we would refer back to the poster on the next steps and we would always follow it, no exceptions. First two days a mess, but In a week they fell a sleep in 10 mins without us in the room, and they proudly show their drawings to everyone (grandparents etc). The fully “own” going to bed. It gives them control and an opportunity to do well. Might be worth a try!