What to do with old stuff.

I would send most of that to auction.

Give the DC their school reports. Burn your own. That's what I've done. DH's s screws will at some point come in useful according to my DH. They are probably safely in the shed or garage. I would leave them to whoever clears out.

The stamps, linen and watercolours should go to auction.

Bin the rest.

then, when they're full, add no more.
🙄 I mean, don't start filling another box!

Suzieque66 Certainly not.

Shops selling vintage items might want the embroidered items if you have one locally.
You could ask the charity shop if they want the paintings, but ask the family first before you dispose of them.

First day covers, not sure, I have a collection from my MIL but yours are more personal so I'd keep those to pass on to family.

Allocate a nice box for each child, fill it with their stuff and things they might want, label it then, when they're full, add no more.

I'd be shredding my school reports!

I would imagine charity shops would want them. DD has been doing one of these feature walls in her living room and she has been buying water colours like those yoy descibenfrom chairty shops. The pillow cases should be very acceptable.

Stamps - contact a reputable stamp dealer and sell them.

Little box - off to the charity shop

Collectors might want them. Have you got a local auction house you could contact? Auctions of have small interesting job lots towards the end. You might not get much, but someone would still get the pleasure of owning them.

Chuck it all away ...