Argh in a small kitchen no jarred sauces!
Your adult kids sound the problem. No wonder they want to stay til they are 30! You buy expensive snacks, find Netflix AND Prime, let them shower for ages, you cook for them, you’re financially compromising yourself for them.
When I first moved out I was always broke - no new clothes or shoes, no car. Staying at home makes it far too easy for them if you fund everything - why would they prefer to stand on their own two feet?
7 minutes in the shower?! I’d be in the garage turning it off at the fuse box (we have a weird electric one). If you can’t do that then tell them, every shower over 2 mins will add a fiver to their rent bill.
Raise the rent to something like what they’d pay in a flat share commercially + realistic share of bills and food so they get used to the outgoings, and save half the rent rise so you have a fund for emergencies or can give them a gift when they do eventually move out.
Also - each adult child makes one family meal per week, they can give you their ingredients list and you add exceptional cost to their rent (so they learn to make cheap food!)
Airfryer probably useless for whole meals for six if you’re using oven anyway (size issue - oven cheaper if you can’t fit whole meal in airfryer) but good for hot snacks like toasted sandwiches or if you have someone using the oven separately due to shifts? Slow cooker is good - get a big one. We have both and I’ve only used the oven twice since the kids went back to school (both times: birthday cake!).
In winter I’d be thinking - dhaal, tomato and lentil soup; vegetable and pearl barley soups; lentil or quorn chilli, chickpea curry, leek and potato soup, stir fries and rice, Thai vegetable curry and rice, cauliflower cheese, macaroni cheese, home-made flat breads and meatballs, tortilla, sausage and bean casserole, poached eggs on toast, beans on toast, liver and onions, cheap meat stewed in the slow cooker, etc.
My mum used to deep-fry potato slices in Yorkshire pudding batter and we’d eat them with baked beans. Or sliced up potato and onion fries until it all caramelises, in sandwiches with salt and vinegar! Very cheap winter food.
Ditch jarred and packet sauces. You’ll do better with tinned tomatoes, a pot of cornflour, brown sugar, cider vinegar, stock cubes, a decent herb and spice rack (I have a bathroom rack on my wall which holds over 30 pots!). Find recipes that use cheap grains for bulk. Healthier and ultimately tastier.
Ditch fizzy drinks and buy a bottle of concentrated squash (Tesco sells Vimto squashes and CapriSun squash which popular with teens visiting my house at least).
With expensive crisps, keep in box under bed and only a few in cupboard to stop mindless grazing.
Ditch biscuits: Cereal is a better snack than biscuits (I think!). Homemade popcorn instead of crisps. Homemade flapjacks.
Lots of fermented foods! They actively promote feeling of being sated.