I’d recommend something that you can put openwrt or opnsense/pfsense on. I think the tplink archers support openwrt at least.
The ISP router opening things at a port level instead of a host level is kinda insane. Do they only support port forwarding? Or when you open a port range can you actually send packets from the WAN to any LAN address at that port.
Can you just buy your own modem, and then also use your own router? (If the reason you need the ISP router is that it also acts as a modem).
Does the ISP router also provide your WiFi? If it does you should definitely go with a second router/access point and then disable the one on the ISP router.
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. This is the opposite of what you’ve suggested, using the upstream ISP router in pass through and relying on the openwrt router to get the ipv6 GUA prefix. (You might even be able to get a larger prefix delegated if you set the settings to ask for it)
If you don’t have bridge mode then things are harder. There’s some helpful information here https://forum.openwrt.org/t/ipv6-only-slaac-dumb-aps/192059/19 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.
Also looking more, the tplink ax55 isn’t supported by openwrt. If you don’t already have it, I’d get something that does. (Or if the default software on the ax55 supports what you want, that’s fine too. I just like having the full control openwrt and similar gives)