That's pretty much the way to go. Keep the ISP modem, but connect it to your own router/firewall and connect your devices to your hardware and not the ISP modem.
So open it up to anyone? DMZ is an open target, not what you want to be doing.
It’s more about protecting your network against a potentially malicious device rather than protecting the device from attackers on the Internet. From that position, placing the isp device on a “DMZ” aka outside your own router/firewall, makes perfect sense.