OP, have you thought of working your way from senior school back? Where do you want them to end up. You don't state if your eldest is a girl or boy or how old they are, where would they go? Would they be boarding at that point?
So just considering your youngest I would consider the following schools (it's the usual suspects tbh).
The Pilgrims probably has about 40% feeding into WinColl. They have a pre-prep and a lovely House mistress when the time comes for your DS to board. Winchester is lovely town with all the amenities you want, close to London and lovely countryside.
I second Aldro which now accepts girls for any siblings. Your main town will probably be Guilford and there is direct quick lines to London too. Lovely countryside setting. Tends to feed into Charterhouse, just across the river.
Elstree is a low key one, I guess. Now accepts girls too with a forest pre-prep. The headmaster used to co-head Ludgrove. A decent amount into Eton, but they mostly feed into Bradfield which is nearby. Lovely countryside town, easy links to London (infact they have a school bus that goes to London Fridays and comes back Monday mornings - at least they used to, have to check it that still the case).
Then you have the usual Oxfordshire and Berks heavy hitters that feed to Eton, WinColl, Radley etc.
Summerfields
Dragons
Ludgrove
Sunningdale
Papplewick
Horris Hil
Going further to the other places you mentioned, have you looked at
Sherborne Prep
Sandroyd
I hope I have not stuck too much into the usual schools OP, and good luck