Lots of good and sensible points raised on here. Op it’s great that you’re doing this research now, but the key thing I think is that you don’t need to make any decisions yet. See how primary goes, get to know what interests and abilities your dc have. Let them make local friends. Save as much money as you can, so you keep your options open.
What nobody has mentioned yet (I don’t think) is it very much depends on the child. They may have strong opinions - they may want to go to the local comp with their friends. Their friends may be a high achieving, studious bunch, or they may be the sort who’ll do anything to be the cool crowd, and good grades may not be cool. This may well influence your decision.
Your dc may be very sporty, and would benefit from the facilities and extra time devoted to sport at a private school. Similarly drama and music - you might decide that the private school having its own theatre is something your kids would get a huge amount from.
But, they may not be interested at all in such extra curricular stuff, in which case you’d be forking out for facilities they aren’t going to appreciate.
Think about their personalities. Are they driven, motivated, hard working? They’ll most likely still get As. Or are they more middling? Do they blossom with more encouragement and nurturing? If so, smaller class sizes where they won’t get lost, especially if they’re ’quiet middles’, could really benefit them.
What does the child want? Visit all the school options. See which is the best fit for the individual child, and their needs. Because it isn’t black and white.