While management traffic is, indeed untagged to the AP, you can place the management traffic where you want it via a native-vlan (aka dual-mode) going out to the AP itself. Between the switches this management VLAN may be carried through various trunks using 802.1q tags via the connected interfaces being TAGGED in the respective VLAN. The router, gateway, or layer-3 switch would have the default-gateway IP and mask for the management subnet carried by said VLAN. The mask would simply size that as a directly-connected network local to the the device with the interface or SVI configured with the ip. I hope that helps.
You can generally place ACLs on layer-3 interfaces, but you have to get the source, destination, protocol, order, and whether it is inbound or outbound correct for the ACLs to work. This used to be hard for me, but after years of doing it, it's very easy now.