If your eldest is in yr5 now, I would look at moving before secondary application cutoff, which is 31st october. It would also give her time to settle and make new friends before going off to secondary.
Also consider the option of School transport for your youngest dc. We moved to an area that was close enough to an out of county specialist school for daily commute on School transport. LAs do like this to be under 45mins for a primary age child, but there is no law dictating how long is too long. If it is the closest school that meets need they will transport, (they will also transport out of county if it is closer, and if the cost of out of county placement plus transport is the cheaper option to in county placement plus transport).
I have 3 dc at 3 different schools, although middle dc already at special, (we wanted specialist) We moved during eldest yr6 and eldest struggled the 1st few weeks of secondary not knowing anyone. We did specifically choose a small secondary, although had we moved slightly earlier we would have had the option of two other larger secondary secondaries, (these were over subscribed, but already ruled out due to size).
I basically got a map out, marked dh work and middle dc specialist school, and smaller secondary school catchments and worked out commutable distances and travel times, which gave us an area in which to look for houses. Once you have your rough area you can start to look at the primary’s and narrow down your options as to which would be suitable for youngest dc.