Yes to a large plastic wallet per subject which can hold the ex book, text book and anything else needed. After homework, everything goes back in that wallet.
Yes to lots of copies of timetable - stuck on wall, by front door, on back of plastic files (see above) and in pencil case and on back of Planner.
Also keep copies of Homework timetable and when homework is due in, in lots of places.
2 loaded pencil cases helps - that way the school one can always stay in the bag and its contents never be left on the desk at home.
Yes to strict training over filling in hwk planner - so they must write the subject above the homework and when it is due in as well as the work. Check every night and insist they do it correctly so you too understand the task. Tick off homework when completed.
If they have lockers, get several keys and keep some yourself.
If they all have the same bag or similar ones, get something identifying such as a keyring or name label.
Useful items to keep in bag are some extra pens as emergency, £10 strictly only for emergencies and £1 for non emergencies such as a cake sale of other charity event. Also a list of crucial phone numbers in case they lose their mobile.
And definitely label all clothes and PE kit if you don't want to constantly buy more.
And get contact nos for other parents in class - some schools issue this if permission is given, if not, try and swap a few details at parents welcome/induction events.