I started with 4 hybrid hens POL in 2011 so a year later than I remembered

but I'm down to 1.5
I say 1.5 because another is looking unwell.
We lost the first one in the first year. She was constantly sickly and treated with flubenvet several times for gape worm. Finally was very depressed and suspected peritonitis due to being egg bound. Taken to vets who also diagnosed this. Given antibiotics and antiinflams but died 3 days later.
The second one laid an egg every day until her last few weeks. I asked about her on here as she had a couple of bouts of ruptured eggs and was worried about her health. She recovered each time then we lost her to suspected peritonitis.
Now another is going the same way.usually lays an egg a day but the last few months has dropped off to one or two a week. I put this down to age thinking she was 3 instead of 2years. In the last month her condition has dropped off drastically. She has a red comb but few eggs. A couple soft shelled and her last two eggs were huge and she was unwell the day before each one. I put this down to the shear size of the eggs.
The last two hens both had bald, sore areas around the back end too as the egg problem started.
They are fed layers pellets with a handful of mixed corn and mealworm at night fresh greens grown for them and dandelion leaves. Oyster shell grit ad lib. Wormed with flubenvet spring and autumn, given poultry spice when moulting. Biodry and diatom used in house after poultry shield sprayed. Dusted with diatom now and
then. Bedding of sawdust and straw in nests. ACV in water once weekly.
Their enclosure is 5m x6m with an apple tree growing in the middle and perches and logs to play on. They have a dustbath with added diatom. There is little/no grass left but they get time out in a pen to nibble now and then. Checking constantly for REDMITE but nothing visible. Have found the odd lice in the past but seem fine at the moment.
To lose two and the third girl looking poor in less than 3 years can anybody pick up what I'm doing wrong?
I'm becoming totally paranoid!
I've reared 5 chicks who are 15 weeks old this week and wanted to start integrating by the beginning of next month but want to be sure they are entering a safe area and that I am capable of looking after them correctly At the moment I feel totally incompetent.
Any help or advice, good or bad, will be appreciated. I just want answers to what I'm doing wrong.