Hey mama,
I'm not a naturally organised person and it's a real effort for me to keep clean (my standards are too high as v.tidy mother & MIL and I give myself a hard time for not keeping up like you also.)
Advice to get rid of most things is great, it helps. I have three boys here and DH so it's a lot.
15 min timer is a good idea just get in a room give it 15 mins full blast sorting out and then leave. Do the next when you have time.
Once it's all done and cleared your head will be in a better place so you can start working out how to keep on top of things.
Hand me downs - bin bag, label throw in attic. Then put a reminder on your phone for when the next DC will want them or you may forget (like me)
Boxes boxes and more boxes. I bought a cheap label maker from home bargains aound £10
Buy boxes for in your larger drawers/ cupboards wherever , label what goes in the boxes and put the box away. It's so much easier if everything has a place.
Eg one of my kitchen cupboards is the home to bits and bobs has the following boxes. Occasion - spare cards, candles, a lighter
House Tools - pliers, tape, annoying screwdriver set for the tiny screw toys etc you get the drift there's also Stationary, Medicines, Covid tests and a few other it's just home to stuff that has no home. My bathroom cupboards have the boxes too - Hair removal, sanitary, daily, hair products, skin care.
It's just easier than chucking everything in.
I know what it's like with the kids and never finding anything.
Use your label maker for DC drawers in their rooms encourage to tidy too. I have labels on the inside most drawers as it makes sense in my head (helpful for DH maybe) even so simple as socks, pyjamas, gym clothes, plain t shirts, it just feels better and easier.
Another tip I try and get into my boys (and myself) - If you walk around and something is on the floor pick it up and don't put it down until it's in the right place. They're forever dumping stuff randomly on surfaces.
Washing - to get on top of my washing if I get behind I sort into bedrooms rather than colour loads. It's much quicker to put away once done as just take the basket to the room and that's it. I had a rota for a bit which worked really well to stay on top of things providing you all have a weeks worth of clothes. M- towels & dc1, T- dc-2, W- dc3, Th- mineand DH & workwear, F - bedding & uniforms.
It's really hard and you're not alone. Sorry this was long