Orbi IP Address Range

I can't really experiment right now, but have you tried changing the subnet mask and then applying the changes? I have the first three octets of the DHCP pool grayed out, and I tried changing the subnet mask to see if the third octet would become editable (it didn't), but I didn't want to apply the changes and disrupt connections. It's possible the DHCP pool is derived from the router's IP address and subnet mask that have been applied, so by changing the IP and/or subnet mask, the portion of the DHCP pool addresses that can be edited will change as appropriate.

Though, I just tested changing the IP address, and the DHCP pool addresses changed immediately, without applying, as soon as I moved the cursor out of the changed octet's field, so maybe not. It might just be lazy programming on their part (maybe even just on the GUI, and the router itself is actualy capable of handling it), and they just assume home users won't need more than 254 IP addresses, so they have it set to copy the first three octets from the IP address to the DHCP pool, regardless of subnet mask. It's possible you can successfully use /23, etc. if you do it from the command line. At least from the web GUI, it seems the third octet in the Orbi's IP address will define the third octet in the DHCP pool.

As for your ISP router allowing 172.16.0.64 - 172.16.255.254, that's a function of the router (and, perhaps, the provisioning by the ISP if it's a modem/router combo unit), but not anything the ISP would be able to restrict on your Orbi. If you had a commercial-grade router, you could use 10.0.0.0 as your network, and your ISP would not know, care, or be able to prevent it.