This is a helpful podcast if you are thinking about what size of kitchen island you want or what to go for: https://shows.acast.com/love-your-kitchen/episodes/exploring-function-over-aesthetics-with-amnda-hughes

Slide and pivot is a neater version of a bifold. Each pane is independent of the other and when you open the first like a door the others slide up to it and then turn outwards. they have much slimmer mullions than bifolds and give you more glass. I have been put off by the very mixed reviews on after sales service from some brands but they do look fantastic.

One option could be to make the sitting room bigger, than you have drawn, to fit a large sofa and creat a corridor behind the stairs with cloaks cupboard, then the kitchen on the left with island peninsular facing the garden and the extensions forming the dining/seating areas

You’ve got the framework which are like carcases. Just ask a carpenter or joiner to cut some MDF to size, buy some hinges and paint the MDF yourselves. Add the hinges to the carcases and the back of the MDF which will be your new doors. Painting the MDF is the cheapest way to go and you can have whatever colour you want. Add handles.