Maybe you can try to downgrade your old firmware. And test again. If the old firmware works well on IPV6, you would better contact tplink support .

gulag wrote 1) C1200 v1 with this firmware is pretty stable, no random reboots. 2) However, IPv6 is still a little quirky. I noticed after a couple of hours, no IPv6 incoming or outgoing connections...

Maybe you need contact tplink support via mail. It seems that is a bug.

As I know C8_V4 support openvon, you can find it at Advanced-VPN Server-OpenVPN I donot know is there any openvpn option on other Archer C8 Hardware Version

It seems that your router management page process crashes, maybe you can simply your network topology to see whether it is related to other devices.

My archer C1200 also limited password length to 15 characters, but I think its enough