@Orangeinmybluelightcup From what you say, DC1 is not working at the level necessary for a grammar school. DC 2 yes.
I can see your concerns about the nearest school. I think you need to drill down with parents about what they don’t like. The school clearly wasn’t good enough before (RI but for what reasons?) but parents now do not, presumably, like the methods being employed to improve the school. However if that’s no chatting, uniform nitpicking, and what seem like draconian changes, it might not be where Dc thrive.
Im amazed any school can possibly be as small as 500. I assume that’s y7-11 only. 90 a year is normal PAN so do they take lots over PAN? Or are they 120 and have places? I don’t believe 500 on roll in a secondary is viable and must severely reduce subjects and other opportunities. I really would have concerns about this. Presumably you know Dc could get in, hence considering it.
Therefore I definitely would find out more about your nearest school. If they are RI for SEN, then they really will be expected to improve. So I might see if you can meet the SENDCO and ask about provision . Also parents often don’t like change so filter what they say. Are their Dc doing well? Are teachers in place to teach the subjects? Are they doing sport, music or clubs a good school should offer? Can they see any improvements?
Open days, that I went to, were very busy with parents. Hundreds and hundreds of them! So extremely difficult to form an opinion of the Dc in a school at all. However art and specialist room displays, general upkeep of school and friendliness of staff might be worth considering. Plus friends nearby might be worth a lot, but that depends on your circumstances: a car and time to get Dc to friends further away.