It is so annoying isn’t it when they change their sleep pattern with no obvious reason for it!
You are doing all the right things and it sounds really frustrating for you. I do have a couple of suggestions that may help fingers crossed.
This one is going to sound bonkers but sometime putting them to bed earlier helps - I notice you said
It may be over tiredness and often an earlier bedtime helps them to slept later.
Check there aren’t any noises waking her. We are more easily woken in the early morning as we are in a lighter phase of sleep. A neighbour leaving for work early, the heating or hot water clicking on and pipes gurgling, or the local milkman on his rounds can all disturb us. In the summer months the dawn chorus can be deafening. If there are disturbances you cannot control you can try white noise which helps to screen out background noise. There are free apps you can download or use an old-fashioned radio not tuned properly – that static is white noise. Pink noise has shown to be better than white noise see link below and is more pleasant to listen to
Other things to think about are to check she is not too cold it can be quite cold in the early morning as the temperature drops. Feeling cold can wake a baby or child. Try using a baby or child safe sleeping bag or blankets and sheets which stay tucked in.
You can try phasing her waking up time - which you have already been doing a bit of. This can work quite well as you gradually start to stretch the time before you go in to her. So, if she is waking at 5am or earlier, don’t go in until 5.10am. Once you have that wake-up time established you can bring it forward gradually by 5-10 minutes until it is a more civilised time. Get her up when you go into her - you may find this may helps her to nap better and for longer in the day.
You can also try scheduled waking, this works well if your child wakes at the same time each morning. The idea of this is you set your alarm to wake her 20 minutes before she usually wake up. You then rouse her gently, so they stir but are not fully awake, and then allow her to settle back to sleep. You do this for a week and then you allow her to sleep through. This method disrupts their natural sleep cycle and resets their body clock so they start to sleep through.
There are a few different options to try - so them each separately and give it a chance to work before trying the next one.
Do let me know how you get on