Yeah openwrt should be great. It uses nftables as a firewall on a Linux distribution. You can configure it through a pretty nice ui, but you also have ssh access to configure everything directly if you want.

The challenge is going to be what the ISP router supports. If it supports bridge mode then things are easy. You just put your router downstream of it and pretend like it’s a modem. Then you configure openwrt like it’s the only router in the network.

If you don’t have bridge mode then things are harder. There’s some helpful information here forum.openwrt.org/t/…/19 even though the situation is slightly different since they also don’t want a firewall. But you probably need to configure your upstream side on the openwrt router similarly.