Yes my router has open ports, but it does not do any port forwarding. So I can 'directly' connect any device behind my router without my router needing to know any specifics of which device that is. And I don't need to do any port forwarding of anything on my network and thus expose them to the whole internet; I just expose them to the users of my tailscale network (only me)
Within my risk appetite on trusted network segments. I have bigger issues if malware is operational within the trust boundary, it can do what it needs using outbound connections just fine (recon, lateral movement, etc). Your risk appetite might differ.
malware. Got any no-name IOT devices on your network? Got some Huawei built hardware anywhere? Playing some new indie game from developers in romania?
I had to install openwrt on my router so that I could restrict access to upnp by mac address just to my gaming pc (imo this should be standard on any router as an advanced setting, most are just upnp yes/no) so that I can still play online games.