Awww that sounds tough. Are there some days your step children don't eat with you? So that would change the requirements?
It sounds just very overwhelming. What might work is if you put together a table with a list of foods along the top with items people will generally eat ice chicken, beef, Quorn, fish, chickpeas, rice, tortillas, gf tortillas, chips, jacket potatoes etc and put each member of the family's name down the side and get them to tick what they will eat ( stress how difficult meal times are and that it's not about what you like, it's about what you are willing to eat).
Then if you look down the columns you at least will have a visual of what foods most people will tollerate, and you can filter, depending on who is present for food.
Then with the ingredients you could pick a style of food - Mexican, Indian, Thai, Italian ...
So if everyone having food will eat rice, broccoli, and chickpeas - you could make a veg Thai curry ( sauce is easy with a curry paste, frozen soffrito,frozen veg and tin of coconut milk) with a big pan of rice and if you have the energy some chicken on the side and a bread and gf bread side.
Or if the vegan isn't around and everyone will eat beef and jacket potatoes - you could do Mexican style chille with jacket spuds and just have some store cupboard/freezer bread style side.
I would rely heavily on freezer foods - frozen veg Inc chopped onions, soffrito etc and marinaded meat you can cook from frozen would be a godsend and store cupboard like tortillas, bake yourself baguettes etc and ready made curry pastes. And if you don't already have one an instant pot, rice cooker and airfryer - not necessarily quicker but once the lid is on, you don't need to prod and poke and don't burn which totally takes the stress out of cooking.
There is nothing wrong with the cooking being your sole responsibility as long as your partner is doing something else to balance.
Good luck. X